This paper proposes a new dynamic preventive control strategy for maintaining power system security, considering both of transient and mid-term stabilities. While the authors have already presented preventive control strategies for transient stability, a new problem has occurred recently regarding mid-term instability in which some generators lose synchronism after 10 second or more from a fault clearance. It becomes clear that the cause of the instability is mainly post-contingency dynamic steady state instability. First, this paper shows a preventive control strategy for improving the mid-term stability using a dominant eigenvalue of post-contingency equilibrium point. Further, this paper presents a unified dynamic preventive control algorithm which can maintain both stabilities together. In this algorithm, the time domain for each stability problem is considered appropriately and the difference between the clasical model and the detailed model is analyzed in detail. The effectiveness of the proposed method is examined by numerical examples for a model system.

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Fuzzy theory has been applied to the thermal power station generating unit maintenance scheduling problem to devise a new method where inherent uncertainty elements are represented appropriately. More specifically, this method uses fuzzy membership functions to represent load demand and supply reserve capacity for each given time interval within the maintenance period and the future years, respectively. The Branch and Bound method is used for the optimisation search.
The basic objective is to equalise the supply reserve capacity using fuzzy membership functions for load demand in the maintenance period, thereby obtaining and averaged value which contains the load's degree of uncertainty. In addtion, membership functions for load and reserve capacity are used to check future year reserve capacity levels to ensure that a given scheduling solution will allow future feasible solutions. To test the effectiveness of this method, simulations were run in tandem with a previously developed method which deals with load reserve capacity values deterministically. For the purpose of comparison, this deterministic approach is also briefly outlined in this paper. The results of these simulations are given in the Numerical Examples section.

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In this paper a new application of neural network for single line outage contingency (screening, evaluation) is proposed. At the contingency screening phase, neural network ANN 1 detects all critical lines whose single outages can cause at least one line overloaded. Then at the contingency evaluation phase, neural network ANN 2 evaluates the outage effect of each critical line on the power flow of other lines. However as the system topology expands and number of lines increases, depending on the existence of irrelevant and redundant data in the input/output patterns of ANN 2 it suffers from, poor convergence performance, long training time and dimensionality. In order to improve these problems, in this paper a line outage oriented power network reduction technique is presented by which the irrelevant and redundant data are eliminated from input/output patterns of ANN 2. Proposed neural networks in conjunction with power network reduction are applied for line outage contigency analysis of IEEE 30-bus (40 lines) system. Simulation results show a promising improvement in the convergence performance and size of neural network ANN 2, compare to the case without network reduction which has a very poor convergence.

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Modeling of transmission towers is an essential part of the traveling wave analysis of lightning surges in transmission lines. In this paper, an equivalent distributed constant line model of the transmission tower is developed. The model consists of three parts: main poles, lattices and crossarms. The surge impedance of each part is expressed by the functions of their dimensions and geometries. This tower model is applied to the 500kV transmission tower of which surge performance characteristics are measured. It is found that the tower voltage wave shapes calculated from this model closely agree with the measured ones. This proves that the authors' proposed tower model well simulates the surge performances of an actual transmission tower.

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A new high-speed digital distance relaying algorithm which uses fast Haar transformation techniques where transformation data window is a half cycle is proposed. In this proposed algorithm, to extract pure power frequency components, a newly designed dc-offset removing digital filter using only 2 adjacent samples is applied because all orthogonal transformations are prone to producing large errors in the presence of exponentially decaying dc-offsets, unless larger data window than one cycle is employed in the execution of the transforms. The prototype digital distance relay is based on a high speed digital signal processing processor utilizing the proposed protection algorithm. In accordance with a series of tests, the relay operating time is from 3[ms] to 11[ms] after faults in a 160 [km], 154 [kV], 60 [Hz] over-head transmission line system.

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A superconducting power transmission system is regarded as being a promising candidate to transmit a bulk electric power through a limited small space. When superconducting cables are introduced as a main transmission line in a future metropolitan electric power system, the large electric power generated in a satellite power plant is directly supplied to the city center through the superconducting cables and is distributed by a conventional underground cable network. In this system configuration, no lightning stroke happens from the sending end of the satellite power plant to the established underground power cable network. The instantaneous bus voltage dip is only induced by a ground fault in the surrounding overhead line area. It is greatly reduced especially in the city center area, because the underground power cable network in the city center area and the overhead line network in the suburban area are connected by a single underground route, and the former is electrically located at a long distance from the latter.
This paper points out that such kind of interferences are reduced so much that more stable power supply is realized in the electric power system with superconducting power transmission system. This is an additional benefit of the introduction of the superconducting power transmission system, and this benefit has been never given from a usual conventional power transmission system. Such kind of additional benefit is expected to compensate the economical weak point of the superconducting power transmission system.

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In design of power systems, we have two big problems, that is, fault current and power system stability, which are reciprocal each other. For each of two problems, we have some countermeasures. As one of the countermeasures, superconducting fault current limiters (ScFCL's) are being developed. By use of FCL's, not only the problems of fault current but also those on power systemstability are expected to be solved.
This paper describes a simple calculation method for stability of power sytems with ScFCL's which may be useful for system designs. Exact considerations led us that ScFCLs are effective not only for suppressing fault current but also for increasing power system stability.

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Performance of a disk-type MHD generator has been investigated. In the experiments, special attention has been devoted to the effects of inlet swirl on generator performance. In order to clarify these effects, 1-dimensional numerical simulation has also been performed. Results showed that enthalpy extraction ratio was successfully improved up to 32.5% by employing inlet swirl with the swirl ratio (=U_θ/U_γ) of 1. Static pressure and Hall potential distributions were measured and showed that this inlet swirl reduced steep increase of static pressure owing to Lorentz force at the inlet of disk channel. Hence relatively low static pressure and high Hall parameter were kept throughout the channel, and therefore, remarkable increase in output performance was achieved. Numerical simulation can confirm qualitatively these experimental results.

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This paper describes developmental research of DC XLPE cable for extra-high voltage use. Through a series of investigations, an XLPE insulation compound containing a 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. Performance results of the prototype cable were very positive. A 250kV DC cable with 20mm insulation thickness was designed and manufactured. Voltage test results showed that the DC insulation characteristics of the cable were excellent. The cable's mechanical performance was verified through a series of tests stipulated for submarine cables. Based on the DC insulation performance analyses, the possibility of reducing the insulation thickness of the developed 250kV DC XLPE cable was investigated, and it was demonstrated that a 14mm walled cable could be used.

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The electric stress inside three-core belted cables is calculated using the charge simulation method combined with the method of imaging. Each conductor is simulated by Nc infinite line charges. An image charge for each fictitious charge is considered to keep the sheath potential at zero. The proposed method has the advantages that the charge values and hence the potential and field stress at any point are expressed as a function of the time wt which result in considerable saving in the computation time. Besides the computation time saving, the proper choice of the values of Nc and the assignment factor fs results in a high accuracy of the potential and field stress. The maximum potential error is shown not to be more than 0.05% and the field deviation angle is not more than 0.11° even when inter-conductor spacing is very small.

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The causes of lightning faults are classified into the direct lightning strokes and induced lightning strokes, which are identified by the characteristics of the lightning overvoltage. In the past, the lightning countermea-sures were mainly directed toward the latter, and attentions have been focused on the installation of arresters, ground wires, and reinforcement of insulators. However, the lightning faults continue to occur, and it is extremely important to clarify the fault characteristice of lightning surges and study the effectiveness of various lightning countermeasure equipment by considering both direct lightning stroke and induced lightning stroke in order to prevent lightning faults in future. In this research, the EMTP has been applied to the direct lightning stroke, and the induced lightning analysis program for multiple conductor systems has been applied to the induced lightning stroke, to study the effectiveness of lightning countermeasures provided by combination of various lightning countermeasure equipments, and we analyzed and evaluated the most effective lightning protection schemes based on verification tests on the full scale models as well as economic considerations.

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Subsonic flows near sonic condition have strong nonlinearities. For example, a slight variation of load condition of a subsonic MHD generator often causes a transition from a subsonic flow near sonic condition into a transonic flow with choking and shock wave. In the present paper the choking phenomena in MHD channels are investigated in order to analyze the nonlinear properties of subsonic MHD generators. The choking point is classified into two kinds by its geometrical property; one of them can appear in the MHD channel, whereas the other cannot appear in the channel and exists at the channel exit. A method of steady state analysis for transonic flow with choking and shock wave considering downstream condition is presented. A steady state analysis of a subsonic diagonal type MHD generator shows that transitions between subsonic flows and transonic flows are often accompanied with bifurcation phenomena such as hysteretic behavior of the flow and jump behavior of the power output.

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This paper presents the V-t characteristics of SF₆ in inhomogeneous field for oscillating transient over-voltage and non-oscillating impulses under positive and negative polarities. The experiments were performed by using a hemisphere-plane arrangement with a needle-shaped protrusion, in the gas pressure range from 0.05 to 0.3 MPa. The predischarge current and the luminous signals were measured by a current shunt and a photomultiplier, respectively. The minimum breakdown voltage with a needle protrusion of 0.5mm in diameter is less than that with a needle protrusion of 1.0mm in diameter. The dependence of breakdown voltage on the diameter of a needle-shaped protrusion is appreciable in the negative polarity and low gas pressure conditions. The formative time-lag to breakdown is influenced by the gas pressure, and the difference between the time-lag for positive and negative polarities increases with the increase of the gas pressure.

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