• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.20 no.10
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• pp.895-900
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• 2007

Partial discharge(PD) test can be performed either when the rotating machine is not operating(off-line) or during normal machine operation(on-line). This paper presents an on-line and off-line PD test on a large hydro-generator and induction motor using the same PD acquisition system(PDAS) and ceramic coupler(CC) sensor. PD signal characteristics of CC sensor proved similar with that of epoxy mica coupler(EMC) sensor as a results of PD test for simulated defect winding and frequency response test. A comparison of on-line and off-line PD test for PD characteristic parameters-phase resolved PD(PRPD), maximum PD value(Qmax) and PD occurrence energy(POE)-indicated that on-line PD test could reliably and effectively diagnose insulation conditions which were verified by off-line PD test.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2010.06a
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• pp.320-320
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• 2010

In the case of existing ultrasonic motors, they have characteristics such as outstanding response speed, speed and high efficiency. However, it's very hard to use practically them as small motors due to complicated structure and expensive cost. This paper proposed v-type ultrasonic linear motor. Stator of the motor is composed of thin elastic body and four ceramics attached to upper and bottom areas of the body. The ceramics have each direction of polarization. When two harmonic voltages which had $90^{\circ}$ phase difference were applied to the ceramics, the symmetric and anti-symmetric displacements were generated at the tip to make the elliptical motion. To find out a model that generates maximum displacement at contact tip, FEM program was used with change of leg-angle. In addition, optimal model was chosen by considering magnitude and shape of displacement according to change of frequency.

• Aerospace Engineering and Technology
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• v.2 no.1
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• pp.108-116
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• 2003

This research presents the design and analysis of PCDU(Power Control & Distribution Unit) of communication satellite. The PCDU of a spacecraft must provide adequate power to each subsystem and payload during mission life, and it also needs high reliability and performance in space environment. A control circuit of the PCDU include bus sensing and filter circuits, error signal amplification circuit, error compensation circuit of SAS(Shunt Assembly Switch) and BPC(Battery Power Converter). The phase margin and DC gain for the designed circuits are analyzed through the frequency response characteristics of the compensated control circuit. And also the transfer function of the battery power converter circuit are discussed at the battery CCCM(Charge Continuous Conduction Mode) and battery C/DCCM(Continuous/Discontinuous Conduction Mode).

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.1
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• pp.31-40
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• 2004

Direct numerical simulations were performed to analyze the effects of time-periodical blowing through a spanwise slot on a turbulent boundary layer. The blowing velocity was varied in a cyclic manner from 0 to 2A$^{+}$(A$^{+}$ =0.25, 0.50 and 1.00) at a fixed blowing frequency of f$^{+}$=0.017. The effect of steady blowing (SB) was also examined, and the SB results were compared with those for periodic blowing (PB). PB reduced the skin friction near the slot, although to a slightly lesser extent than SB. PB was found to generate a spanwise vortical structure in the downstream of the slot. This vortex generates a reverse flow near the wall, thereby reducing the wall shear stress. The wall-normal and spanwise turbulence intensities under PB are increased as compared to those under SB, whereas the streamwise turbulent intensity under PB is weaker than that under SB. PB enhances more energy redistribution than SB. The periodic response of the streamwise turbulence intensity to PB is propagated to a lesser extent than that of the other components of the turbulence intensities and the Reynolds shear stress.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.14 no.5
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• pp.416-423
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• 2004

In this paper, an experimental verification of system identification technique for constructing finite element model is conducted for a three-story test structure equipped with an active mass driver (AMD). Twenty Gaussian white noises were used as the input for AMD, and the corresponding accelerations of each floor are measured. Then, the complex frequency response function (FRF) for the input, the force induced by the AMD, was obtained and subsequently, the Markov parameters and system matrices were estimated. The magnitudes as well as phase of experimentally obtained FRFs match well with those of analytically obtained FRFs.

• Journal of Power Electronics
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• v.11 no.4
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• pp.583-590
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• 2011

Three-level NPC inverters have been put into practical use for years especially in high voltage high power grids. This paper researches three-level active power filters (APFs). In this paper a mathematical model in the d-q coordinates is presented for 3-phase 3-wire NPC APFs. The deadbeat control scheme is obtained by using state equations. Canceling the delay of one sampling period and providing the predictive value of the harmonic current is a key problem of the deadbeat control. Based on this deadbeat control, the predictive output current value is obtained by the state observer. The delay of one sampling period is remedied in this digital control system by the state observer. The predictive harmonic command current value is obtained by the repetitive predictor synchronously. The repetitive predictor can achieve a better prediction of the harmonic current with the same sampling frequency, thus improving the overall performance of the system. The experiment results indicate that the steady-state accuracy and the dynamic response are both satisfying when the proposed control scheme is implemented.

• The Transactions of the Korean Institute of Power Electronics
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• v.17 no.2
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• pp.93-98
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• 2012

A precise measurement of the grid voltage is one of the essential techniques, which is required to connect a renewable energy to the grid. In general, when a filter is used to eliminate unnecessary harmonics and noises, a signal is distorted by phase delay, amplitude attenuation, and other distortions. And the response characteristic of a controller is directly affected by bandwidth of cut-off frequency of the filter. To alleviate this problems, we propose an effective algorithm based on DFT(Discrete Fourier Transform) instead of approaching the filter application. The proposed algorithm ensures high precise measurement of the grid voltage because it can extract the fundamental and harmonics from the raw signal without any distortions. The high performance of the proposed algorithm is verified by PSIM simulation and experiments of Grid-Connected VSI.

• Journal of Power Electronics
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• v.11 no.6
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• pp.914-921
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• 2011

Commencing with incandescent light bulbs, every load today creates harmonics. Unfortunately, these loads vary with respect to their amount of harmonic content and their response to problems caused by harmonics. The prevalent difficulties with harmonics are voltage and current waveform distortions. In addition, Electronic equipment like computers, battery chargers, electronic ballasts, variable frequency drives, and switching mode power supplies generate perilous amounts of harmonics. Issues related to harmonics are of a greater concern to engineers and building designers because they do more than just distort voltage waveforms, they can overheat the building wiring, cause nuisance tripping, overheat transformer units, and cause random end-user equipment failures. Thus power quality is becoming more and more serious with each passing day. As a result, active power filters (APFs) have gained a lot of attention due to their excellent harmonic compensation. However, the performance of the active filters seems to have contradictions with different control techniques. The main objective of this paper is to analyze shunt active filters with fuzzy and pi controllers. To carry out this analysis, active and reactive current methods ($i_d-i_q$) are considered. Extensive simulations were carried out. The simulations were performed under balance, unbalanced and non sinusoidal conditions. The results validate the dynamic behavior of fuzzy logic controllers over PI controllers.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• v.9 no.1
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• pp.947-950
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• 2005

For revolving and elevating a turret of a tank, we substitute an existing oil pressure system with an electric system using a motor and applied the vector control method to this system. A switching method of an inverter for providing desired sinusoidal current to each phase of a motor, we adopted min-max pulse width modulation method which takes less computation time, rather than space vector pulse width modulation method. We designed a digital filter and applied it to the control system. Developed current controller is verified it's performance through a current control test, speed control test, frequency response and tracking a profile of speed test.

• The Transactions of the Korean Institute of Power Electronics
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• v.22 no.2
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• pp.102-108
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• 2017

Proper design guides are proposed for a practical dual-active bridge (DAB) converter based on the mathematical model on the steady state. The DAB converter is popular in bidirectional application due to its zero-voltage capability and easy bidirectional operation for seamless control, high efficiency, and performance. Some design considerations are taken to overcome the limitation of the DAB converter. The practical design methodology of power stage is discussed to minimize the conduction and switching losses of the DAB converter. Small-signal model and frequency response are derived and analyzed based on the generalized average method, which considers equivalent series resistance, to improve the dynamics, stability, and reliability with voltage regulation of the practical DAB converter. The design of closed-loop control is discussed by the derived small-signal model to obtain the pertinent gain and phase margin in steady-state operation. Experimental results of a 3.3 kW prototype of DAB converter demonstrate the validity and effectiveness of the proposed methods.