• Journal of Power Electronics
• /
• 제9권6호
• /
• pp.892-902
• /
• 2009

This paper deals with the dynamic voltage control problem at the inverter end of a HVDC link when connected to a weak AC system which has the potential for harmonic instability and commutation failures. The dynamic voltage control problem is tackled with a STATCOM (Static Compensator), which not only provides a rapid recovery from harmonic instability and commutation failures but also offers a lower cost filter design for HVDC systems. PSCAD/EMTDC simulations are presented to validate the proposed topology and to demonstrate its robust performance.

• Journal of Magnetics
• /
• 제12권3호
• /
• pp.113-117
• /
• 2007

Here we report a theoretical description of ferromagnetic vortex dynamics. Based on Thiele's formulation of the Landau-Lifshitz-Gilbert equation, the motion of the vortex core could be described by a function of the vortex core position. Under a parabolic potential generated in the circular magnetic patterns, the vortex core showed a circular rotation-namely the gyrotropic motion, which could be described by a 2-dimensional simple harmonic oscillator. The gyrotropic frequency and apparent damping constant were predicted and compared with the values obtained micromagnetic calculation.

• 조명전기설비학회논문지
• /
• 제19권1호
• /
• pp.169-174
• /
• 2005

지금까지의 고조파 필터는 중성선 리액터와 지그재그결선의 영상 고조파 전류를 제거하였으나, 부하율에 따라 고조파 저감율이 변하여 경부하에서는 고조파 저감율이 낮아지는 특성이 있고, 중성선 리액터 과열로 중성선 단선 위험의 문제가 발생하였다. 본 Zig-Zag 결선 및 Open Delta 방식을 이용한 새로운 고조파 저감장치(HANOS)는 Zig-Zag 결선의 Core Block내에 추가로 Open Delta 방식을 결선하여 중성선에 연결함으로써 중성선에 흐르는 영상 고조파 전류가 과열 없이 안전하게 제거되는 것을 시험 분석한 결과로 증명하였다.

• Interaction and multiscale mechanics
• /
• 제1권2호
• /
• pp.303-315
• /
• 2008

In this paper, thermodynamical properties of crystalline silicon under strain are calculated using classical molecular dynamics (MD) simulations based on the Tersoff interatomic potential. The Helmholtz free energy of the silicon crystal under strain is calculated by using the ensemble method developed by Frenkel and Ladd (1984). To account for quantum corrections under strain in the classical MD simulations, we propose an approach where the quantum corrections to the internal energy and the Helmholtz free energy are obtained by using the corresponding energy deviation between the classical and quantum harmonic oscillators. We calculate the variation of thermodynamic properties with temperature and strain and compare them with results obtained by using the quasi-harmonic model in the reciprocal space.

• International Journal of Naval Architecture and Ocean Engineering
• /
• 제9권6호
• /
• pp.598-612
• /
• 2017

The recently developed Harmonic Polynomial Cell (HPC) method has been proved to be a promising choice for solving potential-flow Boundary Value Problem (BVP). In this paper, a flux method is proposed to consistently deal with the Neumann boundary condition of the original HPC method and enhance the accuracy. Moreover, fixed mesh algorithm with free surface immersed is developed to improve the computational efficiency. Finally, a two dimensional (2D) multi-block strategy coupling boundary-fitted mesh and fixed mesh is proposed. It limits the computational costs and preserves the accuracy. A fully nonlinear 2D numerical wave tank is developed using the improved HPC method as a verification.

• 대한원격탐사학회지
• /
• 제35권4호
• /
• pp.573-587
• /
• 2019

A multi-temporal approach using remotely sensed time series data obtained over multiple years is a very useful method for monitoring land covers and land-cover changes. While spectral-based methods at any particular time limits the application utility due to instability of the quality of data obtained at that time, the approach based on the temporal profile can produce more accurate results since data is analyzed from a long-term perspective rather than on one point in time. In this study, a multi-temporal approach applying a multi-periodic harmonic model is proposed for classification of remotely sensed data. A harmonic model characterizes the seasonal variation of a time series by four parameters: average level, frequency, phase, and amplitude. The availability of high-quality data is very important for multi-temporal analysis.An satellite image usually have many unobserved data and bad-quality data due to the influence of observation environment and sensing system, which impede the analysis and might possibly produce inaccurate results. Harmonic analysis is also very useful for real-time data reconstruction. Multi-periodic harmonic model is applied to the reconstructed data to classify land covers and monitor land-cover change by tracking the temporal profiles. The proposed method is tested with the MODIS and GOCI NDVI time series over the Korean Peninsula for 5 years from 2012 to 2016. The results show that the multi-periodic harmonic model has a great potential for classification of land-cover types and monitoring of land-cover changes through characterizing annual temporal dynamics.

• Structural Engineering and Mechanics
• /
• 제70권5호
• /
• pp.623-637
• /
• 2019

This paper analyzes the non-stationary vibration and super-harmonic resonances in nonlinear dynamic motion of viscoelastic nano-resonators. For this purpose, a new coupled size-dependent model is developed for a plate-shape nano-resonator made of nonlinear viscoelastic material based on modified coupled stress theory. The virtual work induced by viscous forces obtained in the framework of the Leaderman integral for the size-independent and size-dependent stress tensors. With incorporating the size-dependent potential energy, kinetic energy, and an external excitation force work based on Hamilton's principle, the viscous work equation is balanced. The resulting size-dependent viscoelastically coupled equations are solved using the expansion theory, Galerkin method and the fourth-order Runge-Kutta technique. The Hilbert-Huang transform is performed to examine the effects of the viscoelastic parameter and initial excitation values on the nanosystem free vibration. Furthermore, the secondary resonance due to the super-harmonic motions are examined in the form of frequency response, force response, Poincare map, phase portrait and fast Fourier transforms. The results show that the vibration of viscoelastic nanosystem is non-stationary at higher excitation values unlike the elastic ones. In addition, ignoring the small-size effects shifts the secondary resonance, significantly.

• Journal of the Korean Physical Society
• /
• 제73권11호
• /
• pp.1612-1618
• /
• 2018

In this work, the ground-state properties of an interacting electron gas confined in a two-dimensional quantum dot system with the Gaussian potential ${\upsilon}(r)=V_0(1-{\exp}(-r^2/p))$, where $V_0$ and p are confinement parameters, are determined numerically by using the Thomas-Fermi approximation. The shape of the potential is modified by changing the $V_0$ and the p values, and the influence of the confining potential on the system's properties, such as the chemical energy, the density profile, the kinetic energy, the confining energy, etc., is analyzed for both the non-interacting and the interacting cases. The results are compared with those calculated for a harmonic potential, and excellent agreement is obtained in the limit of high p values for both the non-interacting and the interacting cases.

• 대한전기학회논문지:전기물성ㆍ응용부문C
• /
• 제50권8호
• /
• pp.387-393
• /
• 2001

This paper describes an advanced measuring method and precise evaluation of the ground resistance for the grounding system of energized substations and power equipments. A grounding system of substations consists of all interconnected grounding connections of grounded conductors, neutral ground wires, underground conductors of distribution lines, cable shields, grounding terminals of equipments, and etc. It is very difficult to measure the accurate ground resistance of the grounding terminals of equipments, and etc. It is very difficult to measure the accurate ground resistance of the grounding system of high voltage energized substations because of harmonic components caused by switched power supplies or overloads. The conventional fall-of-potential method may be subject to big error if stray ground currents and potentials are present. In this work, to improve the precision in measurements of the ground resistance by eliminating the effects of harmonic components and stray currents and potentials, the investigations of the ground resistance measurement by using a low pass filter in a model energized grounding system were conducted. The accuracy of ground resistance mesurements was evaluated as a function of the ratio of the test signal to noise (S/N). The errors due to the proposed ground resistance measurement method were decreased with increasing S/N and were less than 5[%] as S/N is 10. The proposed ground resistance measurement method appears to be considerably more accurate than the conventional fall-of -potential method. It is allows cancellation of the parasitic resistance of energized grounding systems, to employ the measurement method that allows cancellation of the parasitic effects due to other circulating ground currents and ground potential rises in practical situations.

• Ocean and Polar Research
• /
• 제33권3호
• /
• pp.309-319
• /
• 2011

This is a preliminary study of the feasibility of obtaining reliable tidal current harmonic constants, using one month of current observations, to verify the accuracy of a tidal model. An inference method is commonly used to separate out the tidal harmonic constituents when the available data spans less than a synodic period. In contrast to tidal constituents, studies of the separation of tidal-current harmonics are rare, basically due to a dearth of the long-term observation data needed for such experiments. We conducted concurrent and monthly harmonic analyses for tidal current velocities and heights, using 2 years (2006 and 2007) of current and sea-level records obtained from the Tidal Current Signal Station located in the narrow waterway in front of Incheon Lock, Korea. Firstly, the l-year harmonic analyses showed that, with the exception of $M_2$ and $S_2$ semidiurnal constituents, the major constituents were different for the tidal currents and heights. $K_1$, for instance, was found to be the 4th major tidal constituent but not an important tidal current constituent. Secondly, we examined monthly variation in the amplitudes and phase-lags of the $S_2$ and $K_1$ current-velocity and tide constituents over a 23-month period. The resultant patterns of variation in the amplitudes and phase-lags of the $S_2$ tidal currents and tides were similar, exhibiting a sine curve form with a 6-month period. Similarly, variation in the $K_1$ tidal constant and tidal current-velocity phase lags showed a sine curve pattern with a 6-month period. However, that of the $K_1$ tidal current-velocity amplitude showed a somewhat irregular sine curve pattern. Lastly, we investigated and tested the inference methods available for separating the $K_2$ and $S_2$ current-velocity constituents via monthly harmonic analysis. We compared the effects of reduction in monthly variability in tidal harmonic constants of the $S_2$ current-velocity constituent using three different inference methods and that of Schureman (1976). Specifically, to separate out the two constituents ($S_2$ and $K_2$), we used three different inference parameter (i.e. amplitude ratio and phase-lag diggerence) values derived from the 1-year harmonic analyses of current-velocities and tidal heights at (near) the short-term observation station and from tidal potential (TP), together with Schureman's (1976) inference (SI). Results from these four different methods reveal that TP and SI are satisfactorily applicable where results of long-term harmonic analysis are not available. We also discussed how to further reduce the monthly variability in $S_2$ tidal current-velocity constants.