• The Transactions of the Korean Institute of Electrical Engineers P
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• v.59 no.2
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• pp.173-178
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• 2010

Frequency is an important operating parameter of a power system. Due to the sudden change in generation and loads or faults in power system, the frequency is supposed to deviate from its nominal value. It is essential that the frequency of a power system be maintained very close to its nominal frequency. And monitoring and an accurate estimation of the power frequency by timing synchronized signal provided by FDR is essential to optimum operation and prevention for wide area blackout. As most conventional frequency estimation schemes are based on DFT filter, it has been pointed out that the gain error by change in magnitude could cause the defects when the power frequency is deviated from nominal value. In this paper, an advanced frequency estimation scheme using gain compensation for fault disturbance recorders (FDR) is presented. The proposed scheme can reduce the gain error caused when the power frequency is deviated from nominal value. Various simulation using both the data from EMTP package and user's defined arbitrary signals are performed to demonstrate the effectiveness of the proposed scheme. The simulation results show that the proposed scheme can provide better accuracy and higher robustness to harmonics and noise under both steady state tests and dynamic conditions.

• Journal of Korean Library and Information Science Society
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• v.44 no.1
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• pp.27-47
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• 2013

This study investigated the classification performance of a kNN classifier using the feature selection methods based on document frequency(DF) and collection frequency(CF). The results of the experiments, which used HKIB-20000 data, were as follows. First, the feature selection methods that used high-frequency terms and removed low-frequency terms by the CF criterion achieved better classification performance than those using the DF criterion. Second, neither DF nor CF methods performed well when low-frequency terms were selected first in the feature selection process. Last, combining CF and DF criteria did not result in better classification performance than using the single feature selection criterion of DF or CF.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.9 no.2
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• pp.237-241
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• 2005

In Bluetooth version 1.0, the frequency hopping algorithm was such that there was one piconet, using a specific frequency, resolving the frequency depending on the part of the digits of the device clock and the Bluetooth address. Basic pattern was a kind of a round-robin using 79 frequencies in the ISM band. At this point, a problem occurs if there were more than two devices using the same frequency within specific range. In this paper, we proposed a software-based adaptive frequency hopping method so that more than two wireless devices can stay connected without frequency crash. Suggested method was implemented with HDL(Hardware Description Language) and automatically synthesized and laid out. Implemented adaptive frequency hopping circuit operated well in 24MHz correctly.

• Journal of The Korean Society of Agricultural Engineers
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• v.46 no.5
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• pp.27-39
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• 2004

In the first part of this study, five homogeneous regions in view of topographical and geographically homogeneous aspects except Jeju and Ulreung islands in Korea were accomplished by K-means clustering method. A total of 57 rain gauges were used for the regional frequency analysis with minimum rainfall series for the consecutive durations. Generalized Extreme Value distribution was confirmed as an optimal one among applied distributions. Drought rainfalls following the return periods were estimated by at-site and regional frequency analysis using L-moments method. It was confirmed that the design drought rainfalls estimated by the regional frequency analysis were shown to be more appropriate than those by the at-site frequency analysis. In the second part of this study, LH-moment ratio diagram and the Kolmogorov-Smirnov test on the Gumbel (GUM), Generalized Extreme Value (GEV), Generalized Logistic (GLO) and Generalized Pareto (GPA) distributions were accomplished to get optimal probability distribution. Design drought rainfalls were estimated by both at-site and regional frequency analysis using LH-moments and GEV distribution, which was confirmed as an optimal one among applied distributions. Design rainfalls were estimated by at-site and regional frequency analysis using LH-moments, the observed and simulated data resulted from Monte Carlotechniques. Design drought rainfalls derived by regional frequency analysis using L1, L2, L3 and L4-moments (LH-moments) method have shown higher reliability than those of at-site frequency analysis in view of RRMSE (Relative Root-Mean-Square Error), RBIAS (Relative Bias) and RR (Relative Reduction) for the estimated design drought rainfalls. Relative efficiency were calculated for the judgment of relative merits and demerits for the design drought rainfalls derived by regional frequency analysis using L-moments and L1, L2, L3 and L4-moments applied in the first report and second report of this study, respectively. Consequently, design drought rainfalls derived by regional frequency analysis using L-moments were shown as more reliable than those using LH-moments. Finally, design drought rainfalls for the classified five homogeneous regions following the various consecutive durations were derived by regional frequency analysis using L-moments, which was confirmed as a more reliable method through this study. Maps for the design drought rainfalls for the classified five homogeneous regions following the various consecutive durations were accomplished by the method of inverse distance weight and Arc-View, which is one of GIS techniques.

• Proceedings of the Korea Institute of Convergence Signal Processing
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• 2000.08a
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• pp.45-48
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• 2000

In this paper algorithms of frequency hopping sequences generation such as Lempel-Greenberger, optimum Lempel-Greenberger and Kumar sequences for spread spectrum communications are described. Using the scalogram based on wavelet transform, time-frequency characteristics of frequency hopped signals corresponding to the considered hopping sequences are analyzed.

• Proceedings of the KSR Conference
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• 2008.06a
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• pp.841-848
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• 2008

This paper examined dynamic characteristics of high speed trains using a time varying frequency transform. Fourier transform based methods are frequently used for the calculation of the dynamic characteristics of trains in the frequency domain, but they cannot represent the time-varying characteristics. Therefore it is necessary to examine their characteristics using a time-varying frequency transform. For the examination, the non-stationary vibration of wheelset, bogie, and carbody are measured using accelerometers and stored in a data aquisition system. They are processed with localization of the data by modulating with a window function, and Fourier transform is taken to each localized data, called the short-time Fourier transform. From the processed results, time varying auto-spectral density, cross-spectral density, frequency response, and coherence functions have been calculated. From the analysis, it is confirmed that the time varying frequency transform is a useful method for analyzing the dynamic characteristics of high speed trains.

• Journal of the Korea Institute of Military Science and Technology
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• v.7 no.2 s.17
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• pp.22-31
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• 2004

In this paper, the measurements preprocessing algorithm for the fading of bearing and frequency measurements is proposed, which can improve the performance of BFTMA(Bearing and Frequency Target Motion Analysis). The fading and detection relation between bearing and frequency are rigorously established for measurements preprocessing, and BFTMA can be carried out the estimation of target motion by using measurements preprocessing. Batch estimation with bearing and frequency using the proposed algorithm can be applied to estimate the initial target states despite of the fading of frequency measurement. Simulation results show that BFTMA using the proposed measurements preprocessing has superior estimation performance, compared with batch estimation using only bearing measurements.

• 전기의세계
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• v.18 no.5
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• pp.20-25
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• 1969

The development of the frequency convertors using semiconductors devices makes it possible to control the speed of A.C. motors easily. It is now economically feasible to provide them with power at adjustable frequency using silicon-controlled rectifier (or thyristor) inverters. In such a case, in order to operate an induction motor efficiently over a wide speed range, it must be supplied from a variable frequency source of which frequency is adjustable over the speed range of the motor. It is desired to observe the changes in characteristics as primary current, torque-speed of induction motor etc. at any optional frequency. Although the characteristics can be obtained by means of the conventional methods, they require very complicated precedures of calculations. The Current Diagram Method in this paper suggests a new approach to simpler calculations of the characteristics, using the motor constants at reference frequency. The conclusions of this study are summarized as follows: 1) The equations of stator current at adjusted frequency were derived to construct graphical chart and the current circle required for the Current Diagram Method. 2) The radius, center of the current circle and the vector locus, the basis for calculating the characteristics, at any desired frequency could be easily determined with the aid of both the derived graphical chart and current circle at reference frequency. 3) The method was shown to be applicable to the various types of 3-phase induction motors and also dealt with its application to the split-phase, condenser motors.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.13 no.4
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• pp.274-280
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• 2003

If a sinusoidal excitation force moves back and forth along a structure with a certain frequency, the structure will be excited with the difference frequency of these two frequencies. A low frequency vibration shaker has been developed using this force frequency shifting without actually moving a shaker The shaker consists of an ordinary eccentric mass shaker, a plate, constant springs, and time varying dampers. The dampers are turned on and off in a sequential manner to simulate a traveling slide of an excitation force. The operation of the shaker is simulated by solving the equations of motion of the shaker. Characteristics of the shaker have been found and they can be utilized to design efficient low frequency shakers.

• Proceedings of the KIEE Conference
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• 2001.07e
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• pp.80-85
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• 2001

Frequency is an important operating parameter of a power system. Frequency of a power system remains constant if sum of all the loads plus losses equals total generation in the system. However, the frequency starts to decrease if total generation is less than the sum of loads and tosses. On the other hand, the system frequency increases if total generation exceeds the sum of loads and losses. Electric power systems sustain transient frequency swings whenever the balance between generation and load does not no longer hold. To cope with this Constraints, it requires an accurate and high speedy frequency deviation estimation technique and suitable adjustment to obtain the power system energy balance. The fundamental frequency component of 3-phase signal is first extracted by using an algorithm based on FIR(finite duration impulse response) filter, a phase angle of a voltage. The rate change of the phase angle is used for estimation and speed in its process. Also, to confirm the validity of the proposed algorithm, the simulation results obtained by using EMTP(electro magnetic transients program) are shown.