• The Transactions of The Korean Institute of Electrical Engineers
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• v.62 no.7
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• pp.905-912
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• 2013

Power system fault analysis is commonly based on well-known symmetrical component method, which describes power system elements by positive, negative and zero sequence impedance. In case of balanced fault, such as three phase short circuit, transmission line can be represented by positive sequence impedance only. The majority of fault in transmission lines, however, is unbalanced fault, such as line-to-ground faults, so that both positive and zero sequence impedance is required for fault analysis. When unbalanced fault occurs, zero sequence current flows through earth and skywires in overhead transmission systems and through cable sheaths and earth in cable transmission systems. Since zero sequence current distribution between cable sheath and earth is dependent on both sheath bondings and grounding configurations, care must be taken to calculate zero sequence impedance of underground cable transmission lines. In this paper, conventional and EMTP-based sequence impedance calculation methods were described and applied to 345kV cable transmission systems (4 circuit, OF 2000mm2). Calculation results showed that detailed circuit analysis is desirable to avoid possible errors of sequence impedance calculation resulted from various configuration of cable sheath bonding and grounding in underground cable transmission systems.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.63 no.10
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• pp.1364-1370
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• 2014

Power system fault analysis has been based on symmetrical component method, which describes power system elements by positive, negative and zero sequence impedance. Obtaining accurate line impedances as possible are very important for estimating fault current magnitude and setting distance relay accurately. Especially, accurate calculation of zero sequence impedance is important because most of transmission line faults are line-to-ground faults, not balanced three-phase fault. Since KEPCO has started measuring of transmission line impedance at 2005, it has been revealed that the measured and calculated line impedances are well agreed within reasonable accuracy. In case of underground transmission lines, however, large discrepancies in zero sequence impedance were observed occasionally. Since zero sequence impedance is an important input data for distance relay to locate faulted point correctly, it is urgently required to analyze, detect and consider countermeasures to the source of these discrepancies. In this paper, development of pre/post processor to ATP (Alternative Transient Program) version of EMTP (Electro-Magnetic Transient Program) for sequence impedance calculation was described. With the developed processor ATP-cable, effects of ground resistance and ECC (Earth Continuity Conductor) on sequence impedance were analyzed.

• The Transactions of the Korean Institute of Power Electronics
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• v.20 no.4
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• pp.321-329
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• 2015

The static synchronous compensator (STATCOM) of cascaded H-bridge configuration accompanying multiple separate DC sides is inherently subject to the problem of uneven DC voltages. These DC voltages in one leg can be controlled by adjusting the AC-side output voltage of each cell inverter, which is proportional to the active power. However, when the phase current is extremely small, large AC-side voltage is required to generate the active power to balance the cell voltages. In this study, an alternative zero-sequence current injection method is proposed, which facilitates effective cell balancing controllers at no load, and has no effect on the power grid because the injected zero sequence current only flows within the STATCOM delta circuit. The performance of the proposed method is verified through simulation and experiments.

• The Transactions of the Korean Institute of Power Electronics
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• v.11 no.2
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• pp.172-183
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• 2006

This paper analyzes instantaneous power compensation theory through comparing p-q theory and cross-vector theory which were proposed by Akagi and Nabae respectively in three-phase four-wire systems. The two compensation theories are identical when there is no zero-sequence voltage component in three-phase three-wire systems, However, when the zero-sequence voltage and/or current components exist in three-phase four-wire systems, the two compensation theories we different in definition on instantaneous real power and instantaneous imaginary power. Based on the analysis, this paper presents instantaneous power compensation method that can eliminate neutral current completely without using energy storage element when the zero-sequence current and voltage components exist in three-phase four-wire systems.

• Journal of Astronomy and Space Sciences
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• v.3 no.1
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• pp.41-51
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• 1986

Two-photometric light curves(Oh and Chen 1984) of the eclipsing binary TX UMa have been analyzed by the method of differential corrections of the model of Wilson and Devinney (1971). The system found to be simi-detached with the cooler and less massive component filling its Roche lobe. The absolute dimensions have been derived from the results of the photometric solutions with the spectroscopic elements of Hiltner(1945). It is assumed that the B8V primary component is on the zero main sequence stage of the core hydrogen burning and the secondary is at the core contraction stage after the shell hydrogen burning stage according to the Iben's (1967) evolutional tracks for $3.0m_\odot$ and $1.0m_\odot$ .

• Proceedings of the KIPE Conference
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• 2012.07a
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• pp.273-274
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• 2012

This paper investigates control algorithms for a doubly fed induction generator(DFIG) with a back-to-back three-level neutral-point clamped voltage source converter in medium voltage wind power system under unbalanced grid conditions. Two different control algorithms to compensate for unbalanced conditions are proposed. Evaluation factors of control algorithm are fault ride-through(FRT) capability, efficiency, harmonic distortions and torque pulsation. Zero regulated negative sequence stator current control algorithm has the most effective performance concerning FRT capability and efficiency. Ripple-free control algorithm nullifies oscillation component of active power and reactive power. Ripple-free control algorithm shows the least harmonic distortions and torque pulsation. Combination of zero regulated negative sequence stator current and ripple-free control algorithm control algorithm depending on the operating requirements and depth of grid unbalance presents the most optimized performance factors under the generalized unbalanced operating conditions leading to high performance DFIG wind turbine system.

• Journal of the Korean Mathematical Society
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• v.33 no.2
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• pp.227-234
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• 1996

Consider nonlinear autoregressive processes of order 1 defined by the random iteration $$ (1) X_{n + 1} = f(X_n) + \epsilon_{n + 1} (n \geq 0) $$ where f is real-valued Borel measurable functin on $R^1, {\epsilon_n : n \geq 1}$ is an i.i.d.sequence whose common distribution F has a non-zero absolutely continuous component with a positive density, $E$\mid$\epsilon_n$\mid$ < \infty$, and the initial $X_0$ is independent of ${\epsilon_n : n > \geq 1}$.

• Proceedings of the KIEE Conference
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• summer
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• pp.480-482
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• 2004

This paper presents a study on the NGR problem for harmonic in distribution systems. Overheating of NGR (Neutral Ground reactor), by neutral current in distribution system, is important cause of transformer breakdown of substation. Countermeasures about zero-sequence component harmonic in neutral line are required.

• Proceedings of the KIEE Conference
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• 2002.11b
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• pp.237-240
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• 2002

본 논문에서는 기존의 무효전력 보상을 정의하고 선로의 영상분 성분을 고려한 순시 유효전력 개념을 도입한 제어 기법을 제안하였다. 이 기법은 3상 4선식 배전 계통에서 영상성분 보상에 효과적임을 알 수 있다. 또한 PSCAD/EMTDC를 이용한 시뮬레이션을 통하여 본 제어 기법이 좋은 성능이 있음을 보였다.

• Proceedings of the Korean Institute of IIIuminating and Electrical Installation Engineers Conference
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• 2003.11a
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• pp.279-281
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• 2003

We analysed the operating properties of resistive type superconducting fault current limiters (SFCLs) based on YBCO thin films with a single line-to-ground fault. When a single line-to-ground fault occurred, the short circuit current of a fault phase increased up to about 6 times of transport currents immediately after the fault instant and was effectively limited to the designed current level within 2 ms by the resistance development of the SFCL. The fault currents of the sound phases almost did not change because of their direct grounding system. The unsymmetrical rates of a fault phase were distributed from 6.4 to 1.4. It was found that the unsymmetrical rates of currents were noticeably improved within one cycle after the fault instant. We calculated the zero phase currents for a single line-to-ground fault using the symmetrical component analysis. The positive sequence resistance was reduced remarkably right after the fault but eventually approached the balanced positive resistance component prior to the system fault. This means that the system reaches almost the three-phase symmetrical state in about 60 ㎳ after the fault. The ground currents were almost 3 times of the zero phase mts since most of the fault currents flowed through the grounding line.