• The Transactions of the Korean Institute of Electrical Engineers
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• v.37 no.10
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• pp.694-701
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• 1988

Distributed-type railguns are designed to maintain the armature current and the length between the armature and the current-feed region nearly constant with time. This paper deals with factors affecting current distribution, effective resistance and effective skin depth in the rail of a distributed-type railgun. Analytical solutions for the current distributions and resistance in the rail are presented for a simple two-dimensional model under steady-state contions. For diffusion limited current, it is found that effective rail resistance is proportional to the square root of the relative velocity, the permeability of the rail and the length between the armature and that effective skin depth of the rail is proportional to the square root of the length and inversely proportional to the square root of the permeability, the conductivity and the velocity.

• 전기의세계
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• v.18 no.6
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• pp.9-22
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• 1969

This thesis has suggested a method of applying the Maximum Principle of Pontryagin to the optimal control of distributive electrical networks. In general, electrical networks consist of branches, nodes, sources and loads. The effective values of steady state currents and voltages are independent of time but only expressed as the functions of position. Moreover, most of the node voltages and branch currents are not predetermined, that is, initially unknown, and their inherent loop characteristics satisfy only Kirchhoff's current and voltage laws. The Maximum Principle, however, needs the initial fixed values of all state variables for its standand way of application. In spite of this inconsistency this thesis has undertaken to suggest a new approach to the successful solution of the above mentioned networks by introducing scaling factors and a state variable change technique which transform the boundary-value unknown problem into the boundary-value partially fixed and partially free problem. For the examples of applying the method suggested, the control problems for minimizing copper quantity in a distribution line have been solved with voltage drop constraint imposed on. In the case of uniform load distribution it has been shown that the optimal wire diameter of the distribution line is reciprocally proportional to the root of distance. For the same load pattern as above the wire diameter giving the minimum copper loss in the distribution line has been shown to be reciprocally proportional to distance.

• Journal of The Korean Society of Agricultural Engineers
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• v.48 no.4
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• pp.41-49
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• 2006

Since particle size distribution curves are useful to estimate permeability of soil, many formulae for permeability coefficient (k) have been published using the parameter from the curves and factors, such as grain size, particle shape and void ratio of soils. However, the parameters such as $C_c,\;C_u$ and $D_n$ derived from only some discrete points on the curve are insufficient to represent the whole gradation. In this paper fractal dimension which is quite new concept and known to be able to represent the entire curve of particle size distribution is employed for the parameters. An empirical formula of permeability coefficient has been developed with fractal dimension and percent of finer than 0.075 mm. The formula developed from this study has confirmed its effectiveness by a series of laboratory tests and comparison to other published formulae. It is found that permeability coefficient is proportional to fractal dimension and inversely proportional to percent of fines.

• Communications for Statistical Applications and Methods
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• v.9 no.1
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• pp.261-270
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• 2002

In this paper, we demonstrate the more accurate confidence intervals for median survival time under the simple proportional hazard model of Koziol and Green (1976) via the Edgeworth expansion for the distribution of the studentized ACL estimator derived in Jeong (2000). The numerical results show that the intervals, so-called test-based and reflect intervals (Slud et al., 1984), outperform normal approximating method in the small sample sizes and/or heavy censoring.

• Journal of the Korean Data and Information Science Society
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• v.5 no.1
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• pp.59-66
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• 1994

Heller and Simonoff(1990) compared several methods of estimating the regression coefficient in a modified proportional hazards model, when the response variable is subject to censoring. We give another method of estimating the parameters in the model which also allows the dependent variable to be censored and the error distribution to be unspecified. The proposed method differs from that of Miller(1976) and that of Buckely and James(1979). We also obtain the variance estimator of the coefficient estimator and compare that with the Buckely-James Variance estimator studied by Hillis(1993).

• The Transactions of The Korean Institute of Electrical Engineers
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• v.58 no.7
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• pp.1342-1348
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• 2009

DSTATCOM(distribution static compensator) is one of the custom power devices, and protects a distribution line from unbalanced and harmonic current caused by non-linear and unbalanced loads. Researches about DSTATCOM are mainly divided two parts, one is the calculation of compensated current and the other part is the current control. This paper proposes a proportional-resonant-repetitive current controller. Improved performance of instantaneous power compensation has been shown by simulations and experiments.

• Journal of the Korean Statistical Society
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• v.36 no.1
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• pp.129-148
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• 2007

Frailty estimates from the proportional hazards frailty model often lead us to conjecture the heterogeneity in frailty such that the variance of the frailty varies over different covariate groups (e.g. male group versus female group). For such systematic heterogeneity in frailty, we consider a regression model for the variance components in the proportional hazards frailty model, denoted by the MLFM. However, in many cases, the observed data do not show any statistically significant preference between the homogeneous frailty model and the heterogeneous frailty model. In this paper, we propose a Bayesian model averaging procedure with the reversible jump Markov chain Monte Carlo which selects the appropriate model automatically. The resulting regression coefficient estimate ignores the model uncertainty from the frailty distribution in view of Bayesian model averaging (Hoeting et al., 1999). Finally, the proposed model and the estimation procedure are illustrated through the analysis of the kidney infection data in McGilchrist and Aisbett (1991) and a simulation study is implemented.

• Tunnel and Underground Space
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• v.8 no.2
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• pp.79-86
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• 1998

This study was performed to suggest to suggest suitable design conditions of water curtain system through analysis on pressure down in boreholes by hydraulic tests carried out I construction fields for underground oil storages. The influence by hydraulic conductivities of rock mass around boreholes on pressure down in boreholes was analysed. The relationship between array of boreholes and their pressure down was also analysed. Groundwater flow analysis on crude oil and LPG storages was carried out to evaluate results of field tests and to investigate distribution of hydraulic gradient in rock mass around cavern using finite difference method. As the results, hydraulic tests showed that pressure down in boreholes was inverse proportional to the hydraulic conductivity of surrounding rock mass. The rate of pressure down of boreholes was not influenced by water curtain system more than 20m over cavern and was proportional to installation interval of boreholes. The hydraulic gradient in rock mass around cavern was proportional to distance and interval of boreholes and its value was not satisfactory to oil tightness condition in case of no water curtain system.

• International Journal of Reliability and Applications
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• v.2 no.1
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• pp.1-26
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• 2001

The purpose of this paper is to introduce a proportional reversed hazard rate model, in contrast to the celebrated proportional hazard model, and study some of its structural properties. Some criteria of ageing are presented and the inheritance of the ageing notions (of the base line distribution) by the proposed model are studied. Two important data sets are analyzed: one uncensored and the other having some censored observations. In both cases, the confidence bands for the failure rate and survival function are investigated. In one case the failure rate is bathtub shaped and in the other it is upside bath tub shaped and thus the failure rates are non-monotonic even though the baseline failure rate is monotonic. In addition, the estimates of the turning points of the failure rates are provided.

• Journal of Power Electronics
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• v.18 no.3
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• pp.757-765
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• 2018

This paper deals with the instability and resonant phenomena in distribution systems with multiple grid-connected inverters with an LCL output filter. The penetration of roof-top and other types of small photovoltaic (PV) grid-connected systems is rapidly increasing in distribution grids due to the attractive incentives set forth by different governments. When the number of such grid-connected inverters increases, their interaction with the distribution grid may cause undesirable effects such as instability and resonance. In this paper, a grid system with several grid-connected inverters is studied. Since proportional-resonant (PR) controllers are becoming more popular, it is assumed that most inverters use this type of controller. An LCL filter is also considered at the inverters output to make the case as realistic as possible. A complete modeling of this system is presented. Consequently, it is shown that such a system is prone to instability due to the interactions of the inverter controllers. A modification of PR controllers is presented where the output capacitor is virtually decreased. As a result, the instability is avoided. Simulation results are presented and show a good agreement with the theoretical studies. Experimental results obtained on a laboratory setup show the validity of the analysis.