• Title, Summary, Keyword: Low frequency oscillation

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Characteristics of Low-Frequency Combustion-driven Oscillation in a Surface Burner (표면연소기의 저주파 연소진동음의 특성)

  • 한희갑;이근희;권영필
    • Journal of KSNVE
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    • v.10 no.6
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    • pp.991-997
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    • 2000
  • The objective of this study is to examine the onset condition and the frequency characteristics of the low-frequency combustion oscillation in a surface burner. For this purpose, extensive parametric studies have been performed experimentally and the effects of size of each section, the equivalence ratio, and the entrance velocity on oscillatory behavior explored. The experimental results were discussed in comparison with the other combustors associated tilth the low-frequency combustion oscillation. The combustion mode is driven at high combustion rate by the lift of unstable flame near the lower limit of the combustible equivalence ratio. The oscillation frequency is dependent not on the burner geometry but on the equivalence ratio and the combustion load. Low-frequency combustion mode was formed to be divided into two different modes, named C1 and C2 respectively. Two modes occurred individually, simultaneously or transitionally according to the equivalence ratio and combustion load. The characteristics of low-frequency oscillation is different from each other depending on the type of combustors. The surface burner has also its own characteristics of low -frequency oscillation.

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Research of the Mechanism of Low Frequency Oscillation Based on Dynamic Damping Effect

  • Liu, Wenying;Ge, Rundong;Zhu, Dandan;Wang, Weizhou;Zheng, Wei;Liu, Fuchao
    • Journal of Electrical Engineering and Technology
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    • v.10 no.4
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    • pp.1518-1526
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    • 2015
  • For now, there are some low frequency oscillations in the power system which feature low frequency oscillation with positive damping and cannot be explained by traditional low frequency oscillation mechanisms. Concerning this issue, the dynamic damping effect is put forward on the basis of the power-angle curve and the study of damping torque in this article. That is, in the process of oscillation, damping will dynamically change and will be less than that of the stable operating point especially when the angle of the stable operating point and the oscillation amplitude are large. In a situation with weak damping, the damping may turn negative when the oscillation amplitude increases to a certain extent, which may result in an amplitude-increasing oscillation. Finally, the simulation of the two-machine two-area system verifies the arguments in this paper which may provide new ideas for the analysis and control of some unclear low frequency phenomena.

A New DPWM Method to Suppress the Low Frequency Oscillation of the Neutral-Point Voltage for NPC Three-Level Inverters

  • Lyu, Jianguo;Hu, Wenbin;Wu, Fuyun;Yao, Kai;Wu, Junji
    • Journal of Power Electronics
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    • v.15 no.5
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    • pp.1207-1216
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    • 2015
  • In order to suppress the low frequency oscillation of the neutral-point voltage for three-level inverters, this paper proposes a new discontinuous pulse width modulation (DPWM) control method. The conventional sinusoidal pulse width modulation (SPWM) control has no effect on balancing the neutral-point voltage. Based on the basic control principle of DPWM, the relationship between the reference space voltage vector and the neutral-point current is analyzed. The proposed method suppresses the low frequency oscillation of the neutral-point voltage by keeping the switches of a certain phase no switching in one carrier cycle. So the operating time of the positive and negative small vectors is equal. Comparing with the conventional SPWM control method, the proposed DPWM control method suppresses the low frequency oscillation of the neutral-point voltage, decreases the output waveform harmonics, and increases both the output waveform quality and the system efficiency. An experiment has been realized by a neutral-point clamped (NPC) three-level inverter prototype based on STM32F407-CPLD. The experimental results verify the correctness of the theoretical analysis and the effectiveness of the proposed DPWM method.

Numerical investigation of film boiling heat transfer on the horizontal surface in an oscillating system with low frequencies

  • An, Young Seock;Kim, Byoung Jae
    • Nuclear Engineering and Technology
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    • v.52 no.5
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    • pp.918-924
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    • 2020
  • Film boiling is of great importance in nuclear safety as it directly influences the integrity of nuclear fuel in case of accidents involving loss of coolant. Recently, nuclear power plant safety under earthquake conditions has received much attention. However, to the best of our knowledge, there are no existing studies reporting film boiling in an oscillating system. Most previous studies for film boiling were performed on stationary systems. In this study, numerical simulations were performed for saturated film boiling of water on a horizontal surface under low frequencies to investigate the effect of system oscillation on film boiling heat transfer. A coupled level-set and volume-of-fluid method was used to track the interface between the vapor and liquid phases. With a fixed oscillation amplitude, overall, heat transfer decreases with oscillation frequency. However, there is a frequency region in which heat transfer remains nearly constant. This lock-on phenomenon occurs when the oscillation frequency is near the natural bubble release frequency. With a fixed oscillation frequency, heat transfer decreases with oscillation amplitude. With a fixed maximum amplitude of the additional gravity, heat transfer is affected little by the combination of oscillation amplitude and frequency.

Reignition of Methanol Droplet Flames Under Acoustic Pressure Oscillation (메탄올 액적 화염의 음향파 가진에 의한 재점화)

  • Kim, Hong Jip;Sohn, Chae Hoon;Chung, Suk Ho
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.23 no.1
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    • pp.114-122
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    • 1999
  • Reignition as special cases of acoustic pressure responses of flame are numerically studied by employing methanol droplet flame as a laminar flamelet. Quasi-steady flame responses occur in the range of small amplitude, low frequency oscillation. Reignition phenomena can occur when, by increasing the frequency of large amplitude acoustic pressure, the magnitude of characteristic acoustic time is the same order of that of characteristic reaction time of flames. And more increasing of amplitude of acoustic pressure induces the direct extinction of flame. Flame can sustain its own intensity even under the steady extinction temperature in case of high frequency acoustic oscillation, and this tendency is remarkable with increasing frequency. Reignition regime with respect to amplitude and frequency of acoustic pressure doesn't exist in low frequency($10^2$ Hz, in this study), but broadens with frequency of acoustic pressure.

Initial Results of Low Frequency Oscillation Analysis based on Synchrophasor in KEPCO System (Synchrophasor를 이용한 한전계통의 저주파 진동 해석 초기 결과)

  • Shim, Kwan-Shik;Kim, Sang-Tae;Nam, Hae-Kon;Choi, Joon-Ho
    • The Transactions of The Korean Institute of Electrical Engineers
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    • v.63 no.1
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    • pp.1-9
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    • 2014
  • The parameters of electromechanical modes offer considerable insight into the dynamic stability properties of a power system. This paper describes an initial result for extraction of dynamic parameters from synchrophasor measurements collected on the KEPCO system. Dominant modes of the system are estimated by oscillation detecting program in K-WAMS. The critical wide-area modes of KEPCO system have frequencies in the 0.4 to 0.7Hz range. And the local mode causing the low frequency oscillation of generators located on the west coast area has in the frequency 0.97 and 1.25 Hz, respectively. This results can serve as a reference in the future for reliable system operation in KEPCO system.

Damping of Low Frequency Oscillation in Power System using Robust Control of Superconductor Flywheel Energy Storage System (초전도 플라이휠 에너지 저장장치의 강인제어를 이용한 전력계통의 저주파진동 억제)

  • Lee, J.P.;Kim, H.G.
    • Progress in Superconductivity
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    • v.14 no.1
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    • pp.52-59
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    • 2012
  • In this paper, the robust superconductor flywheel energy storage system(SFESS) controller using $H_{\infty}$ control theory was designed to damp low frequency oscillation of power system. The main advantage of the $H_{\infty}$ controller is that uncertainties of power system can be included at the stage of controller design. Both disturbance attenuation and robust stability for the power system were treated simultaneously by using mixed sensitivity $H_{\infty}$ problem. The robust stability and the performance for uncertainties of power system were represented by frequency weighted transfer function. To verify control performance of proposed SFESS controller using $H_{\infty}$ control, the closed loop eigenvalue and the damping ratio in dominant oscillation mode of power system were analyzed and nonlinear simulation for one-machine infinite bus system was performed under disturbance for various operating conditions. The results showed that the proposed $H_{\infty}$ SFESS controller was more robust than conventional power system stabilizer (PSS).

Eigenvalue Analysis and Detection of Low Frequency Oscillation using PMU Data in KEPCO System (위상동기신호를 이용한 한전계통의 저주파진동 검출과 고유치해석)

  • Shim, Kwan-Shik;Kim, Sang-Tae;Kim, Tae-Kyun;Ahn, Seon-Ju;Choi, Joon-Ho
    • 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.

The Oscillation Characteristics of a Magneticfluid Plug in Curved Tube (곡관내 자성유체 PLUG의 진동특성)

  • Chun, U.H.;Lee, H.N.
    • Transactions of the Korean Society of Automotive Engineers
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    • v.3 no.3
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    • pp.46-57
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    • 1995
  • The aim of the present study is to provide fundamental informations for the development of magneticfluid actuator. To achieve the aim, oscillation characteristics of the magneticfluid plug are investigated by experiment for the various length and position of the magneticfluid plug and the frequency of magnetic field. The oscillation characteristics are obtained. Amplitude, natural frequency, phase shift and damping ratio, are compared with theoretical values. From the study, the following conclusive remarks can be made. The experimental equation for the magnetic field is obtained. The critical magneticfluid length exists and its value is about 70mm. The range of the damping ratio and fluid loss coefficient obtained by experiment are 0.1~0.2 and 30~100, respectively. Comparison between experimental and theoretical results of oscillation characteristics shows good agreement in the high frequency range. Meanwhile, in the low frequency range, there appears little discrepancies(5% in the frequency and amplitude and 10% in phase difference and damping ratio) with each other.

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Analysis and Control of Low Frequency Oscillation using TCSC Small Signal Model by Control of Firing Angles (TCSC의 소신호 모형을 이용한 점호각 제어에 의한 저주파 진동 감쇠 효과 해석 및 제어)

  • Kim, Tae-Hyun;Seo, Jang-Cheol;Park, Jong-Keun;Moon, Seung-Ill;Han, Byung-Moon
    • Proceedings of the KIEE Conference
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    • pp.120-124
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    • 1995
  • TCSC can not only increase power flow but also damp low frequency oscillation by controlling firing angles of thyristors. But, a model considering voltage, current firing angles is not derived. This paper used a small signal model considirng these variables which was derived in paper [1]. TCSC model is combined with swing equation. Being related to rotor angles and firing angles of thyristors, current and synchronizing torque coefficient is reformulated. Because firing angles of thyristors can be controlled only twice within one period, swing equation is transformed to discrete time model. It is shown that low frequency oscillation can be damped by controlling firing angles in one machine infinite bus power system.

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