• Journal of Electrical Engineering and Technology
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• v.5 no.3
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• pp.503-510
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• 2010

This paper proposes the use of fuzzy applications to a 4-machine and 10-bus system to check stability in open conditions. Fuzzy controllers and the excitation of a synchronous generator are added. Power system stabilizers (PSSs) are added to the excitation system to enhance damping during low frequency oscillations. A fuzzy logic power system stabilizer (PSS) for stability enhancement of a multi-machine power system is also presented. To attain stability enhancement, speed deviation ($\Delta\omega$) and acceleration ($\Delta\varpi$) of the Kota Thermal synchronous generator rotor are taken as inputs to the fuzzy logic controller. These variables have significant effects on the damping of generator shaft mechanical oscillations. The stabilizing signals are computed using fuzzy membership functions that are dependent on these variables. The performance of the fuzzy logic PSS is compared with the open power system, after which the simulations are tested under different operating conditions and changes in reference voltage. The simulation results are quite encouraging and satisfactory. Similarly, the system is tested for the different defuzzification methods, and based on the results, the centroid method elicits the best possible system response.

• Journal of Electrical Engineering and Technology
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• v.5 no.2
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• pp.197-208
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• 2010

Unified power flow controller (UPFC) is the most reliable device in the FACTS concept. It has the ability to adjust all three control parameters effective in power flow and voltage stability. In this paper, a linearized model of a power system installed with a UPFC has been presented. UPFC has four control loops that by adding an extra signal to one of them, increases dynamic stability and load angle oscillations are damped. In this paper, after open loop eigenvalue (electro mechanical mode) calculations, state-space equations have been used to design damping controller and it has been considered to influence active and reactive power flow durations as the input of damping controller, in addition to the common speed duration of synchronous generators as input damper signal. To increase stability, further Lead-Lag and LQR controllers, a novel on-line adaptive controller has been used analytically to identify power system parameters. Closed-loop calculations of the electro mechanical mode verify the improvement of system pole placement after controller designing. Suitable operation of adaptive controller to decrease rotor speed oscillations against input mechanical torque disturbances is confirmed by the simulation results.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.47 no.10
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• pp.720-727
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• 2019

Resonating thermal lags of solid fuel with heat transfer oscillations generated by boundary layer oscillation is the primary mechanism of the occurrence of the LFI (Low Frequency Combustion Instability) in hybrid rocket combustion. This study was experimentally attempted to confirm that how the boundary layer was perturbed and led to the LFI. Special attention was also made on oxidizer swirl injection to investigate the contribution to combustion stabilization. Also the overall behavior of fluctuating boundary layer flow and the occurrence of the LFI was monitored as swirl intensity increased. Fluctuating boundary layer was successfully monitored by the captured image and POD (Proper Orthogonal Decomposition) analysis. In the results, oscillating boundary layer became stabilized as the swirl intensity increases. And the coupling strength between high frequency p', q' diminished and periodical amplification of RI (Rayleigh Index) with similar frequency band of thermal lag was also decreased. Thus, results confirmed that oscillating axial boundary layer triggered by periodic coupling of high frequency p', q' is the primary mechanism to excite thermal resonance with thermal lag characteristics of solid fuel.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.1682-1685
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• 2004

Gas Turbine combustors for power plant can be reduced NOx emissions using lean premixed combustion technology. But the combustors are likely to occur combustion oscillations which damage operation reliability and mechanical life of the gas turbines. In this paper, characterizations of oscillation in a gas turbine combustor for power plant are presented. Combustion dynamics occur $1{\sim}1.5$ psi in amplitude with low frequency less than 140Hz during normal operation. An abnormal high level dynamics, 2.0 psi amplitude occur at 125 Hz frequency. Abnormal combustion oscillation is reduced by modulation of fuel supply valve control schedule.

• Journal of Mechanical Science and Technology
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• v.19 no.spc1
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• pp.227-236
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• 2005

Multibody system dynamics is based on classical mechanics and its engineering applications originating from mechanisms, gyroscopes, satellites and robots to biomechanics. Multibody system dynamics is characterized by algorithms or formalisms, respectively, ready for computer implementation. As a result simulation and animation are most convenient. Recent developments in multibody dynamics are identified as elastic or flexible systems, respectively, contact and impact problems, and actively controlled systems. Based on the history and recent activities in multibody dynamics, recursive algorithms are introduced and methods for dynamical analysis are presented. Linear and nonlinear engineering systems are analyzed by matrix methods, nonlinear dynamics approaches and simulation techniques. Applications are shown from low frequency vehicles dynamics including comfort and safety requirements to high frequency structural vibrations generating noise and sound, and from controlled limit cycles of mechanisms to periodic nonlinear oscillations of biped walkers. The fields of application are steadily increasing, in particular as multibody dynamics is considered as the basis of mechatronics.

• The Transactions of the Korean Institute of Electrical Engineers A
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• v.49 no.5
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• pp.226-232
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• 2000

This paper presents application results of the augmented matrix eigen-sensitivity theories to TCSC siting problem for damping the inter-area low frequency oscillation in the large KEPCO system. First and second-order eigen-sensitivities of the inter-area low frequency oscillation in the large KEPCO system. First and second-order eigen-sensitivities of the inter-area mode are computed fro changes in susceptance of the transmission lines. The lines having high sensitivity are chosen as the initial candidates for installing TCSC. Then for each of the chosen candidates, Bodeplot of the transfer function with line susceptance as the input and the bus voltage at one side of the line as the output is computed. Using the Bode plots, the lines having any zeros near the inter-area mode are screened out since design of TCSC controller is very difficult in such a case. The $H_{\infty}$ TCSC controller installed at any finally chosen candidate is found to be effective in damping the inter-area oscillation, and the proposed TCSC siting algorithm is proved to be valid. Design of $H_{\infty}$ controller is described in Part IIof this paper.

• International Journal of Fluid Machinery and Systems
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• v.4 no.1
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• pp.179-190
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• 2011

The swirling flow developing in Francis turbine draft tube under part load operation leads to pressure fluctuations usually in the range of 0.2 to 0.4 times the runner rotational frequency resulting from the so-called vortex breakdown. For low cavitation number, the flow features a cavitation vortex rope animated with precession motion. Under given conditions, these pressure fluctuations may lead to undesirable pressure fluctuations in the entire hydraulic system and also produce active power oscillations. For the upper part load range, between 0.7 and 0.85 times the best efficiency discharge, pressure fluctuations may appear in a higher frequency range of 2 to 4 times the runner rotational speed and feature modulations with vortex rope precession. It has been pointed out that for this particular operating point, the vortex rope features elliptical cross section and is animated of a self-rotation. This paper presents an experimental investigation focusing on this peculiar phenomenon, defined as the upper part load vortex rope. The experimental investigation is carried out on a high specific speed Francis turbine scale model installed on a test rig of the EPFL Laboratory for Hydraulic Machines. The selected operating point corresponds to a discharge of 0.83 times the best efficiency discharge. Observations of the cavitation vortex carried out with high speed camera have been recorded and synchronized with pressure fluctuations measurements at the draft tube cone. First, the vortex rope self rotation frequency is evidenced and the related frequency is deduced. Then, the influence of the sigma cavitation number on vortex rope shape and pressure fluctuations is presented. The waterfall diagram of the pressure fluctuations evidences resonance effects with the hydraulic circuit. The influence of outlet bubble cavitation and air injection is also investigated for low cavitation number. The time evolution of the vortex rope volume is compared with pressure fluctuations time evolution using image processing. Finally, the influence of the Froude number on the vortex rope shape and the associated pressure fluctuations is analyzed by varying the rotational speed.

• Advances in materials Research
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• v.1 no.4
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• pp.245-268
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• 2012

Results of isothermal torsional oscillation tests are reported on melts of linear low density polyethylene and isotactic polypropylene. Prior to rheological tests, specimens were annealed at various temperatures ranging from $T_a$ = 180 to $310^{\circ}C$ for various amounts of time (from 30 to 120 min). Thermal treatment induced degradation of the melts and caused pronounced decreases in their molecular weights. With reference to the concept of transient networks, constitutive equations are developed for the viscoelastic response of polymer melts. A melt is treated as an equivalent network of strands bridged by junctions (entanglements and physical cross-links). The time-dependent response of the network is modelled as separation of active strands from and merging of dangling strands with temporary nodes. The stress-strain relations involve three adjustable parameters (the instantaneous shear modulus, the average activation energy for detachment of active strands, and the standard deviation of activation energies) that are determined by matching the dependencies of storage and loss moduli on frequency of oscillations. Good agreement is demonstrated between the experimental data and the results of numerical simulation. The study focuses on the effect of molecular weight of polymer melts on the material constants in the constitutive equations.

• Structural Engineering and Mechanics
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• v.89 no.6
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• pp.589-599
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• 2024

This study presents the usability of the high-rate single-frequency Precise Point Positioning (SF-PPP) technique based on 20 Hz Global Positioning Systems (GPS)-only observations in detecting dynamic motions. SF-PPP solutions were obtained from post-mission and real-time GNSS corrections. These include the International GNSS Service (IGS)-Final, IGS real-time (RT), real-time MADOCA (Multi-GNSS Advanced Demonstration tool for Orbit and Clock Analysis), and real-time products from the Australian/New Zealand satellite-based augmentation systems (SBAS, known as SouthPAN). SF-PPP results were compared with LVDT (Linear Variable Differential Transformer) sensor and single-frequency relative positioning (SF-RP) solutions. The findings show that the SF-PPP technique successfully detects the harmonic motions, and the real-time products-based PPP solutions were as accurate as the final post-mission products. In the frequency domain, all GNSS-based methods evaluated in this contribution correctly detect the dominant frequency of short-term harmonic oscillations, while the differences in the amplitude values corresponding to the peak frequency do not exceed 1.1 mm. However, evaluations in the time domain show that SF-PPP needs high-pass filtering to detect accurate displacement since SF-PPP solutions include trends and low-frequency fluctuations, mainly due to atmospheric effects. Findings obtained in the time domain indicate that final, real-time, and MADOCA-based PPP results capture short-term dynamic behaviors with an accuracy ranging from 3.4 mm to 8.5 mm, and SBAS-based PPP solutions have several times higher RMSE values compared to other methods. However, after high-pass filtering, the accuracies obtained from PPP methods decreased to a few mm. The outcomes demonstrate the potential of the high-rate SF-PPP method to reliably monitor structural and earthquake-induced ground motions and vibration frequencies of structures.

• The Transactions of the Korean Institute of Electrical Engineers A
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• v.52 no.7
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• pp.371-380
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• 2003

In this paper, a new design method of H$H_\infty$-Qn PSS using genetic algorithm(GA) is proposed to efficiently damp low frequency oscillations despite the uncertainties and various disturbances of power systems. The selection method of evaluation function is proposed for selecting the robust PSS parameters. All QFT boundaries are satisfied automatically and H$H_\infty$-norm is minimized simultaneously without trial and error procedure. The eigenvalues and the damping ratio of dominant oscillation mode are investigated to evaluate performance of designed controller for one machine infinite bus system. A disturbance attenuation performance is investigated through singular value bode diagram of the system. Dynamic characteristics are considered to verify robustness of the proposed PSS by means of nonlinear simulations under various disturbances for various operating conditions. The results show that the proposed PSS is more robust than conventional PSS.