Author show that in the low-voltage power system, the grounding electrode resistance (R_e) and the system grounding resistance (R_N) are ruled by concerned power system bank capacity P(kVA). So by reasons of safety, protection and economy, it leads the following formula P(R_e+R_N)=const. Still more, he studies that super low resistance type (R_e=under 25Ω) gives an optimum solution for industrial low voltage power sytem by means of theories and examples.
More, it is effective to suppressing of surges and noises concerning to power system etc. Author show it by some reports and experiences.
And author analyze the universal suitable ability for grounding standard of main countries in recent subject.

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Transient stability analysis is one of the most important studies related to power systems. As the scale of the power system grows, the computational effort required for this analysis increases greatly. Use of equivalents to represent certain parts of the system is a popular approach to overcome the above difficulty. Coherency based equivalents requiring the identification of coherent machines is one of widely used methods.
In this paper, a new efficient method is proposed to characterize coherent behavior of machines in power systems. The linearized set of system equations is employed to describe the dynamics of the machines which are departing from their equilibrium state. In the proposed method a theorem and a corollary are derived using the idea of perfect coherency and modal analysis. Later, a minimization technique in an inner product space is incorporated in this analysis in order to extend the idea of perfect coherency to the idea of practical coherency which is designated as close coherency throughout this paper. The practicality of the close coherency is further diversified introducing the idea of global and local coherency. Finally numerical examples are used to give a better picture of the proposed approach.

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Construction of dynamic equivalents plays an important role in the effort to reduce the computational burden of the transient stability analysis of power systems. Coherency based one is an attractive dynamic equivalent for power systems.
In this paper, the authors propose a new efficient practical algorithm for the identification of coherent machine groups in large-scale power systems. In construction of this algorithm, two coherency conditions which are derived from the theorem presented in the authors' previous paper are used as the tools. Several prerequisites for such an algorithm are also discussed. Two test system are analyzed for the grouping of coherent machines, under various fault locations within the system, using the proposed method. In order to prove the validity of this algorithm, results obtained by standard transient stability simulations under same conditions are used to check the accuracy of the results obtained by the proposed method. Later, summarizing the results, the variation of coherent groups as a result of change in the study system is demonstrated.

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Owing to the growing size and complexity in power systems, system analysis, such as the transient calculation, take much time hence fast calculation methods are required. Although the parallel processing is a hopeful method, there was a difficulty in the parallel solution of the linear equation which appears in the power-flow calculation through the Newton-Raphson method.
This paper aims at the fast calculation of the power-flow ploblem via parallel processing. In order to improve the suitability to the parallel solution of the differential equation in transient calculation, we assume the use of a direct-mapping-type parallel processing machine, which directly maps the network of a power system onto the network of processors. Under this assumption, we propose a new parallel-processing-oriented method, where the linear equation is solved by linear-iterations between nodes with the Aitken acceleration. We simulate the method on three model power systems and compare this Parallel Iterative Method (PIM) with a Parallel Direct Method (PDM), which uses the banded matrix, according to the number of operations required. As the results, we can expect that the PIM may solve the linear equations faster than the PDM with m processors, although the PIM might be inferior to the PDM with mxm processors, where m denotes the halfband-width of the banded matrix.

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The problem of estimating fault section is suitable for application of expert systems. In fact, many papers have been published on expert systems for this problem, but most systems use production rule for representing experts' knowldge. The use of production rule results in a system with simple knowledge representation. However, since there are many devices in power systems, a great number of rules must be described and it is difficult to check the knowledge bases.
This paper presents a new method, called the protective system simulation and describe a developed practical expert system using it. The method gives protective coordination to the assumed fault sections. Protective coordination is used for judging whether relays operate correctly, or not. It is based on the ontology, which is the basic concept of qualitative reasoning. We classified knowledge concerning estimating fault sections into two regions, non-teleological and teleological knowledge.
The method uses protection zone and time setting for representing the function of each relay. The expert system based on the protective system simulation has well maintainability and infers fault section within reasonable time. We plan to build this system into three actual control centers in 1991.

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A new hierarchical dynamic technique for multi-area power system state estimation is proposed by means of the goal coordination method. In designing various control schemes for large scale power systems, it is indispensable to decompose the system model and coordinate the optimization results in each subsystem. While several approaches based on the model coordination method were reported, the authors have developed a hierarchical structure of static estimation, in which the goal coordination method is applied. For this algorithm, it should be noted that the estimates after coordinating are the same as the results obtained by the integrated state estimation basically. In the dynamic case, however, since the decomposition of the objective function becomes complicated, the above distinctive feature is not necessarily retained. Although the coordination between an upper level and subsystems is approximated one, the performance of the proposed method is expected to be superior to the static estimation because of including the information at the previous time sample.

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This paper proposes an approximate impedance formula of a conductor with an arbitrary cross-section, and also a method of approximating the conductor by an equivalent circular conductor. Thus, a multiconductor system consisting of arbitrary cross-section conductors is reprenented by circular cross-section conductors of which the impedance and admittance are evaluated by the existing impedance and admittance formulas.
The calculated results by the proposed method agree satisfactorily with measured results and numerical results by a finite element method which is, at the present, only the way of calculating such the impedance, but requires a large computation time and memories. The proposed method might be very useful to evaluate the impedance and admittance of a multiconductor system consisting of arbitrary cross-section conductors.

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The authors propose a new power consolidation-inversion-control system for the Faraday MHD generator using the voltage source PWM inverters. The d. c. output power from each electrodes pair of MHD generator is at first inverted into three phase a. c. power by voltage source PWM inverter and then the a. c. powers are consolidated by transformers. The proposed system does not need expensive equipments of a. c. filter and phase modifier and can independently and simultaneously control the active and reative powers provided to the electric power system. Numerical simulations of the whole system including the Faraday MHD generator, the proposed power consolidation-inversion-control system and the electric power system show that the proposed system can stably and steadily transmit and control the electric power from MHD gnerator to electric power system. It is also confirmed that the proposed system can independently and simultaneously control the active and reactive powers and can be used as a fast power controller.

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A doubly fed induction generator is useful to local use water power plant. Setting it in non constant water flow, we can use it a a constant frequency power generator.
This paper shows how to control this generator system.
This paper has two important points. One is, this generator system is always exited at primaly wdnding. Another is, our control target is 2ndary q axis current to get constant voltage of primary generated power.

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Considering that the electrical conductivity of a thermally equilibrium MHD plasma is a strong non-linear function of the gas temperature, the authors first pointed out that the statistically averaged conductivity <σ> should be evaluated by the mean temprature as <σ>=(1+G)σ(<T>), where G is given by the third order Taylor expansion with respect to the temperature fluctuation T-<T> and is always positive. Next, in order to obtain a statistically averaged Ohm's law for a turbulent plasma, the correlation <σ′E′> between the conductivity and the electric field fluctuations has been numerically investigated for a plasma with a 1/7th-power average temperature distribution and layered one-dimensional fluctuation between parallel electrodes. It was shown that the correlation is always negative and the averaged current density as well as the plasma resistance is to be corrected appreciably from the values based on the mean temperature. Finally, it was shown that the correction factor <σ′E′>/<σ><E> can be evaluated approximately by a 4th-order polynominal of the relative rms temperature fluctuation √<T′²>/<T> and the relationship is practically insensitive to the variation of the electrode temperature, the boundary-layer thickness and also to the spatial distribution of the fluctuation amplitudes.

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