• Proceedings of the Korea Water Resources Association Conference
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• 2011.05a
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• pp.90-90
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• 2011

Long-term nonstationary oscillations (NSOs) are commonly observed in hydrological and climatological data series such as low-frequency climate oscillation indices and precipitation dataset. In this work, we present a stochastic model that captures NSOs within a given variable. The model employs a data-adaptive decomposition method named empirical mode decomposition (EMD). Irregular oscillatory processes in a given variable can be extracted into a finite number of intrinsic mode functions with the EMD approach. A unique data-adaptive algorithm is proposed in the present paper in order to study the future evolution of the NSO components extracted from EMD.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.57 no.3
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• pp.363-368
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• 2008

In this paper, the Resistive Companion Form(RCF) analysis method is applied to analyze small signal stability of power systems including thyristor controlled FACTS equipments such as SVC. The eigenvalue sensitivity analysis algorithm in discrete systems based on the RCF analysis method is presented and applied to the power system including SVC. As a result of simulation, the RCF analysis method is proved very effective to precisely calculate the variations of eigenvalues or newly generated unstable oscillation modes after periodic switching operations of SVC. Also the eigenvalue sensitivity analysis method based on the RCF analysis method enabled to precisely calculate eigenvalue sensitivity coefficients of controller parameters about the dominant oscillation mode after periodic switching operations in discrete systems. These simulation results are different from those of the conventional continuous system analysis method such as the state space equation and proved that the RCF analysis method is very effective to analyze the discrete power systems including periodically operated switching equipments such as SVC.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.27 no.12
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• pp.63-73
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• 2013

The parameters of electromechanical modes offer considerable insight into the dynamic stability properties of a power system. This paper presents a results of a LFO(low-frequency oscillation) based on the time-synchronized signals measured by synchrophasor in the rolling blackout. Spectral analysis was performed, and critical parameters were estimated using the data acquired from synchrophasors installed in the KEPCO system. As significant modes, a 0.68 Hz oscillation mode that occurred prior to the forced load shedding in the rolling blackout was estimated. Such an oscillation mode can cause an uncontrollable blackout. Therefore, the system should be operated so that significant oscillation modes are not activated. This results can serve as a reference in the future for reliable system operation in the event of a similar blackout.

• Atmosphere
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• v.21 no.2
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• pp.143-149
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• 2011

A new multi-timescale analysis method, Ensemble Empirical Mode Decomposition (EEMD), is used to diagnose the variation of the MJO activity determined by 850hPa and 200hPa zonal winds from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis data for the 56-yr period from 1950 to 2005. The results show that MJO activity can be decomposed into 9 quasi-periodic oscillations and a trend. With each level of contribution of the quasi-periodic oscillation discussed, the bi-seasonal oscillation, the interannual oscillation and the trend of the MJO activity are the most prominent features. The trend increases almost linearly, so that prior to around 1978 the activity of the MJO is lower than that during the latter part. This may be related to the tropical sea surface temperature(SST). It is speculated that the interdecadal change in the MJO activity appeared in around 1978 is related to the warmer SST in the equatorial warm pool, especially over the Indian Ocean.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2006.05a
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• pp.279-282
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• 2006

Experiments in low strain rate methane-air counterflow diffusion flames diluted with $CO_2$ have been conducted to investigate the flame extinction behavior and edge flame oscillation The critical mole fraction at flame extinction is examined in terms of velocity ratio and global strain rate. Onset conditions of the edge flame oscillation and the relevant modes are also provided with global strain rate. It is observed that flame length is intimately relevant to lateral heat loss, and this affects flame extinction and edge flame oscillation considerably. Edge flame oscillations are categorized into three: a growing-, a decaying-, and a harmonic-oscillation mode.

• Journal of Welding and Joining
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• v.11 no.2
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• pp.62-73
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• 1993

Weld pool oscillation for the full-penetration GTA welding process was investigated for its possible application to weld penetration control through theoretical modeling and experiment. Energy method was used to estimate the natural frequency of the molten pool having the physically-acceptable weld geometry and oscillation modes. An unique experimental system was built which had the data acquisiton and video capabilities so that the pool oscillation signals and molten pool surfaces could be monitored continuously. Pool oscillation was detected through arc voltage and arc light emission simultaneously. The signal from arc light emission showed good coherence with that from arc voltage, and arc light generated the higher quality signal. The molten pool was found to oscillate in different oscillation modes based on the travel speed and weld geometry. The natural frequency estimated from the theoretical model agreed reasonably well with the experimental results.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.66 no.2
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• pp.261-284
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• 2017

This paper describes the results of a low-frequency oscillation analysis using data measured in PMU installed in the KEPCO system, and the comparison with eigenvalues computed from the linear model. The dominant oscillation modes are estimated by applying various algorithms. The algorithms are: the extended Prony method; multiple time interval parameter estimation method; subspace system identification method; and spectral analysis. From the measurement data, modes of frequency 0.68[Hz] and 0.92[Hz] were estimated, and modes of frequency 0.63[Hz] and 0.80[Hz] were computed from the eigenvalue calculation. There was a difference between the mode estimated from measurement data and that from the linear model. This is possibly because of an error in the dynamic data of the KEPCO system used in eigenvalue calculation. Because wide area modes exist in the KEPCO system, these modes should be monitored continuously for the reliable operation of the system. In order to prevent total blackouts caused by wide area oscillation, moreover, contingency analysis should be performed in relation to this mode and appropriate measures should be established.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.21 no.2
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• pp.126-130
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• 2009

The present study describes an investigation into the characteristics of combustion oscillation and its suppression instability of a silo type gas turbine combustor in commercial power plant. Combustion oscillation is occurred the combustor in near full load during operation. As a result of FFT analysis of the combustion dynamics, the frequency of the oscillation is analyzed as the 1'st longitudinal mode of acoustic resonance of the combustor. For suppress of the instability, combustion tuning with adjust of fuel valve schedule is carried out, which changes equivalent ratio of each burners. As the result, the oscillation is successfully reduced with meeting the level of NOx emission regulation.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1997.04a
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• pp.621-626
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• 1997

Combustion instability is a common phenomenon in a ducted flame burner and is known as accompanying low frequency oscillation. This is due to the interaction between unsteady heat release rate and sound pressure field, that is, thermoacoustic feedback. In Rayleigh criterion, combustion instability is triggered when the heat additions is in phase with acoustic oscillation. A Rijke type burner with a pre-mixed flame is built for investigating the effect of Reynolds number and equivalence ratio on thermoacoustic oscillation. In addition, the effect of wall temperature is presented. The results suggest that the frequency of max. oscillation is dependent on Reynolds number and equivalence ratio whereas its magnitude is not a strong function of these two parameters. On the other hand, the wall temperature distribution has much strong effects on the oscillation, even creates different mode of acoustic resonance.

• Proceedings of the KIEE Conference
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• 2002.11b
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• pp.167-170
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• 2002

This paper presents a mode estimation for the analysis of small signal stability in power system. The low frequency oscillation mode estimation is based on Prony method that is able to accurately compute the modal parameters (frequency and damping) of oscillation mode from time series. The time series or time domain data is obtained in TSA process. The method applied to a large scale power systems and compared on the eigenanalysis results.