Snow accretion on transmission lines in heavy snowfall areas may cause critical accidents such as tower collapses, sleet jumps, etc., There are several deicing countermeasures of loading large currents to melt snow or ice on transmission lines by dissipating heats. To assist power system operators in making plan of deicing countermeasures against these accidents, a prototype expert system had been developed. The system forecasts snow accretion on transmission lines from meteorological data and power system operational data and makes a list of all the feasible and effective deicing countermeasures.
As an effort to construct a more practical system, this paper analyzes the simulation results, which are made by using the meteorological data collected from the real accidents caused by snow accretion on transmission lines in southern part of Hokkaido area during past 10 years. The simulation results show the rulebase of the system is very effective and they can be improved further by introduction of fuzzy inference. An approach to generate the membership function of the fuzzy production rules and to analyze the fuzzy inference result is proposed too.

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Authors defined that the optimum location of an intermediate switching stations in bulk power long distance transmission system is the points where the transient stability is at an average level, from the viewpoint that margin of transient stability varies depending upon the local load and location of fault on the lines. To make this location more useful, fault location and variation of local loads shall be taken into account.
This paper introduces the probabilistic stability index and examines the optimal location of switching stations as follows: (1) The stability probability was extended to handle fault location in continuous manner, istead of discrete way. (2) The optimization index was proposed, which takes the probability with fault location as random variable into account; and the optimum location were analyzed. (3) Considering the time dependence of fault occurrence and load variation, a procedure for evaluation of fault occurrence probability which depends on the load was proposed and its effectiveness was verified. (4) The optimum location of switching station was examined with variation of loads taken into account. To make the optimum location more practical is made possible in this sense.

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This paper proposes a method for determining a parameter to caluculate the most critical eigenvalue in the S-matrix method. The QR method has been widely utilized due to the high efficiency. Since it needs calculating all the eigenvalue to judge if power systems are stable or not, it is not easily applicable to real-sized systems for lack of storage. In recent years, the S-matrix method was developed to make the calculation easier. The method utlizes mapping the eigenvalue from s-plane to z-plane. Thus, the method enables us to focus on the most critical eigenvalue. However, it has a problem that a mapping parameter is determined by trials and errors. This paper presents a guideline for determining the parameter. The method is based on the Gershgorin's theorem that specifies the eigenvalue region. The proposed method has been applied to sample systems such as 3-unit 9-bus and 10-unit 39-bus systems. The effectiveness of the method is demonstrated.

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This paper discusses the feasibility of a decentralized voltage control scheme for large scale power systems. The algorithm is intended to apply to reactive normal operating state from emergency state caused by ill-condition and also to keep the operating state away from approaching unstable boundaries in preventive control. The procedure for releasing voltage deviation (over/under voltage) is formulated as a multi-stage decision process over a certain time interval. The optimization problem is transformed into a two-point boundary value problem using the discrete maximum principle and is solved easily by using the discrete Riccati equation. For a large-scale power system, control values need to be computed from a large number of state variables, and this inevitably prolongs the slow dynamics with controllers. The centralized control system in a large-scale system cannot be justified from the economical and technical points of view. To resolve above problems, we present a decentralized voltage control system incorporating slow voltage dynamics.

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This paper presents criteria for evaluating flexibility of a generation mix and develops an efficient computational algorithm for determining a robust generation mix considering uncertainties in the power system expansion planning. The robust generation mix problem is formulated as a multi-objective optimization problem and solved by the dynamic programming technique in which each type of generation plants is selected as a stage and generation capacity is selected as a state. The proposed method can treat easily not only the uncertainties but also many constraints of generation planning such as integer solutions of generation capacities, reliability constraints, and so on. The effectiveness and feasibility of the proposed method are demonstrated on a typical power system model.

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The purpose of this paper is to make clear the influence of phase difference on ferrite orthogonal core (FOC). The flux control characteristic and flux distribution of FOCs which have phase difference between the primary and the secondary flux are examined. Flux control performance of FOC as a measuring control element is requested by resolving control characteristic of each flux components from the flux distribution.

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In Tokyo Electric Power Company, we are advancing the digitalization of the protection relay to improve the performance of power system protection and to free us from troubulesome maintenance of protection relay. At present, around a half of the whole relays used in 500kV and 275kV power systems are microprocessor-based relays, so-called digital type relays, and in the other power systems around one third are digital type relays. Therefore we require high level reliability of digital type relay and it is now in higher level than the conventional solid state relay's.
This paper describes the classification of the hardware failure of digital type relay and also shows the effective methods to improve its reliability. Methods for reducing the failure and the bad influence on the performance of the relay are introduced. It is found that aging test of the relay in higher temperature and heat cycle test in severer condition are very effective to be reappeared the failure that does not occur continuously and to find out the failure component. And as a result, we are able to reduce the failure rate of the components made from semiconductors such as digital type IC by applying these testing methods to screening test of the components.

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