• Journal of Electrical Engineering and Technology
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• v.4 no.4
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• pp.451-456
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• 2009

Through the development of phasor measurement units (PMU), various aspects of power system dynamic behavior could be monitored and diagnosed. Monitoring dynamic voltage stability has become one of the achievements we can obtain from PMUs. It is very important to select the most appropriate method for the Korea Electric Power Corporation (KEPCO) system since there are many voltage stability indices. In this paper, we propose an advanced WAVI (Wide Area Voltage Stability) that is well suited for the purposes of monitoring the dynamic voltage stability of KEPCO's PMU installation plan. The salient features of the proposed index are: i) it uses only PMU measurements without coupling with EMS data; ii) it is computationally unburdened and can be applied to real-time situations. The proposed index is applied to the KEPCO test system and the results show that it successfully predicts voltage instability through comparative studies.

• Korean Journal of Materials Research
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• v.16 no.8
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• pp.466-472
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• 2006

The electrical, dielectric properties, and its stability of ZPCCL-based varistors were investigated for different sintering temperatures in the range of $1230{\sim}1300^{\circ}C$. As the sintering temperatures increased, the varistor voltage decreased in the range of $777.9{\sim}108$ V/mm, the nonlinear coefficient decreased in the range of $77.9{\sim}7.1$, and the leakage current increased in the range of $0.3{\sim}50.6\;{\mu}A$. The stability of electrical and dielectric characteristics was obtained from sintering temperature of $1260^{\circ}C$. the varistors sintered at $1260^{\circ}C$ marked the high electrical and dielectric stability, with $%{\Delta}{V_{1mA}=+1.9%,\;%{\Delta}{\alpha}=-10.6%,\;%{\Delta}I_L=+20%\;and\;%{\Delta}tan\;{\delta}=+9.9%$ for DC accelerated aging stress state of $0.95V_{1mA}/150^{\circ}C$/24 h.

• Korean Journal of Materials Research
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• v.17 no.6
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• pp.298-302
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• 2007

The electrical properties and its stability of Zn-Pr-Co-Cr-Tb-based varistors were investigated for different sintering temperatures. As the sintering temperatures increased, the varistor voltage decreased in the range of $705.2{\sim}299.1$ V/mm, the nonlinear coefficient decreased in the range of $42.4{\sim}31.7$, and the leakage current was in the range of $1.0{\sim}1.7\;{\mu}A$. The stability of electrical characteristics increased with the increase of sintering temperature. The varistors sintered at $1350^{\circ}C$ marked the high electrical stability, with $%\Delta$ $V_{1mA}=+0.1%,\;%\Delta{\alpha}=+3.2%$, and $%{\Delta}I_L=+117.6%$ for DC accelerated aging stress state of $0.95V_{1mA}/150^{\circ}C/24\;h$.

• Journal of Electrical Engineering and Technology
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• v.13 no.5
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• pp.1798-1806
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• 2018

When a contingency occurs in a large transmission route in a power system, it can generate various instabilities that may lead to a power system blackout. In particular, transient instability in a power system needs to be immediately addressed, and preventive measures should be in place prior to fault occurrence. Measures to achieve transient stability include system reinforcement, power generation restriction, and generator tripping. Because the interpretation of transient stability is a time domain simulation, it is difficult to determine the efficacy of proposed countermeasures using only simple simulation results. Therefore, several methods to quantify transient stability have been introduced. Among them, the single machine equivalent (SIME) method based on the equal area criterion (EAC) can quantify the degree of instability by calculating the residual acceleration energy of a generator. However, method for generator tripping effect evaluation does not have been established. In this study, we propose a method to evaluate the effect of generator tripping on transient stability that is based on the SIME method. For this purpose, the measures that reflect generator tripping in the SIME calculation are reviewed. Simulation results obtained by applying the proposed method to the IEEE 39-bus system and KEPCO system are then presented.

• Journal of Electrical Engineering and Technology
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• v.12 no.2
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• pp.541-548
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• 2017

An increasing concern is paid to short-term voltage stability with the growth of penetration of induction motor loads. Reactive power reserve(RPR) of power system is critical to improve voltage stability. A definition of short-term voltage stability-related RPR(SVRPR) is proposed. Generators vary their contributions to voltage stability with their location and system condition, etc. Voltage support coefficient based on the second-order trace sensitivity method is proposed to evaluate SVRPR's contribution to short-term voltage stability. The evaluation method can account for the generator's reactive support in transient process and the contingency severity. Then an optimization model to improve short-term voltage stability is built. To deal with multiple contingencies, contingency weight taking into account both its probability and severity is proposed. The optimization problem is solved by primal dual interior point method. Testing on IEEE_39 bus system, it is indicated that the method proposed is effective. Short-term voltage stability is improved significantly by the way of SVRPR optimization. Hence, the approach can be used to prevent the happening of voltage collapse during system's contingency.

• Proceedings of the KIEE Conference
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• 1998.07c
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• pp.979-981
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• 1998

This paper presents a stability proof for the nonlinear feedback linearization-observer/sliding mode model following controller (NFL-O/SMMFC). The separation principle is derived, and the closed-loop stability is proved by a Lyapunov function candidate using an addition form of the sliding surface vector and the estimation error.

• Journal of Electrical Engineering and Technology
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• v.8 no.2
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• pp.197-205
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• 2013

Real time transient stability assessment mainly depends on real-time prediction. Unfortunately, conventional techniques based on offline analysis are too slow and unreliable in complex power systems. Hence, fast and reliable stability prediction methods and simple stability criterions must be developed for real time purposes. In this paper, two new methods for real time determining critical clearing time based on clustering identification are proposed. This article is covering three main sections: (i) clustering generators and recognizing critical group; (ii) replacing the multi-machine system by a two-machine dynamic equivalent and eventually, to a one-machine-infinite-bus system; (iii) presenting a new method to predict post-fault trajectory and two simple algorithms for calculating critical clearing time, respectively established upon two different transient stability criterions. The performance is expected to figure out critical clearing time within 100ms-150ms and with an acceptable accuracy.

• Journal of Electrical Engineering and Technology
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• v.8 no.4
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• pp.694-703
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• 2013

In electric power system, the line stability indices adopted in most of the instances laid stress on variation of reactive power than real power variation of the transmission line. In this paper, a proposal is made with the formulation of a New Voltage Stability Index (NVSI) which originates from the equation of a two bus network, neglecting the resistance of transmission line, resulting in appreciable variations in both real and reactive loading. The efficacy of the index and fuzzy based load flow are validated with IEEE 30 bus and Tamil Nadu Electricity Board (TNEB) 69 bus system, a practical system in India. The results could prove that the identification of weak bus and critical line in both systems is effectively done. The weak area of the practical system and the contingency ranking with overloading either line or generator outages are found by conducting contingency analysis using NVSI.

• Journal of Power Electronics
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• v.13 no.5
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• pp.896-908
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• 2013

Under high grid impedance conditions, it is difficult to guarantee the stability of grid-connected inverters with an LCL filter designed based on ideal grid conditions. In this paper, the theoretical basis for output impedance calculation is introduced. Based on the small-signal model, the d-d channel closed-loop output impedance models adopting the converter-side current control method and the grid-side current control method are derived, respectively. Specifically, this paper shows how to simplify the stability analysis which is usually complemented based on the generalized Nyquist stability criterion (GNC). The stability of each current-controlled grid-connected system is analyzed via the proposed simplified method. Moreover, the influence of the LCL parameters on the stability margin of grid-connected inverter controlled with converter-side current is studied. It is shown that the stability of grid-connected systems is fully determined by the d-d channel output admittance of the grid-connected inverter and the inductive component of the grid impedance. Experimental results validate the proposed theoretical stability analysis.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2001.11b
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• pp.472-475
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• 2001

The electrical properties and stability of ZPCCY-based varistors composing of $ZnO-Pr_{6}O_{11}-CoO-Cr_{2}O_{3}-Y_{2}O_{3}$ ceramics were investigated with sintering time. As sintering time is increased, the nonlinear exponent decreased in the range of 51.19~26.70. Among varistors having above 30 in nonlinear exponent, for the varistor sintered for 1h, the nonlinearity was superior to the stability comparatively and, in the case of 2h, the stability was superior to the nonlinearity relatively. Consequently, it is estimated that the varistors sintered for 1~2h will be applied to various fields by trade-off between nonlinearity and stability.