• Economic and Environmental Geology
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• v.49 no.4
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• pp.269-280
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• 2016

I presented three vector crustal magnetic anomaly components and six gradients by using spherical Slepian functions over the cap area of $20^{\circ}$ of radius centered on the South Pole. The Swarm mission, launched by European Space Agency(ESA) in November of 2013, was planned to put three satellites into the low-Earth orbits, two in parallel in East-West direction and one in cross-over of the higher altitude. This orbit configuration will make the gradient measurements possible in North-South direction, vertical direction, as well as E-W direction. The gravity satellites, such as GRACE and GOCE, have already implemented their gradient measurements for recovering the accurate gravity of the Earth and its temporal variation due to mass changes on the subsurface. However, the magnetic gradients have little been applied since Swarm launched. A localized magnetic modeling method is useful in taking an account for a region where data availability was limited or of interest was special. In particular, computation to get the localized solutions is much more efficient and it has an advantage of presenting high frequency anomaly features with numbers of solutions fewer than the global ones. Besides, these localized basis functions that were done by a linear transformation of the spherical harmonic functions, are orthogonal so that they can be used for power spectrum analysis by transforming the global spherical harmonic coefficients. I anticipate in scientific and technical progress in the localized modeling with the gradient measurements from Swarm and here will do discussion on the results of the localized solution to represent the three vector and six gradient anomalies over the Antarctic area from the synthetic data derived from a global solution of the spherical harmonics for the crustal magnetic anomalies of Swarm measurements.

• Journal of Astronomy and Space Sciences
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• v.15 no.1
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• pp.139-150
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• 1998

We measured the terrestrial nightglow of OI $6300{\AA}$ in the thermosphere (~250km) using a ground-based Fabry-Perot interferometer at the King Sejong Station, Antarcticfrom March through September, 1997. The King Sejong Station is located at high latitude geographically ($62.22^{\circ}S,\;301.25^{\circ}E$) but at mid latitude geomagnetically ($50.62^{\circ}S,\;7.51^{\circ}E$). It is therefore the strategic location to measure the temperatures of the thermosphere in the Southern Hemisphere associated with both solar and geomagnetic activities. In this study, we analyzed the observed temperatures inrelation to F10.7 and Kp indices to examine the effect of the solar and the geomagneticactivities on high-latitude neutral thermosphere. During the observing period, the solaractivity was at its minimum. The measured temperatures are usually in the range between about 600~1000 K with some seasonal variation and are higher than those predicted by semi-empirical model, VSH (Vector Spherical Harmonics) and empirical model, MSIS (Mass-Spectrometer-Incoherent-Scatter)-86.

• Journal of Power Electronics
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• v.12 no.1
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• pp.145-156
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• 2012

These days, the application of electronic power transformers (EPTs) is expanding in place of ordinary power transformers. These transformers can transmit power via three or four wire converters. Their dynamic performance is extremely important, due to their complex structure. In this paper, a new method is proposed for improving the dynamic performance of distribution electronic power transformers (DEPT) by using sliding mode control (SMC). Hence, to express the dynamic characteristics of a system, different factors such as the voltage unbalance, voltage sag, voltage harmonics and voltage flicker in the system primary side are considered. The four controlling aims of the improvement in dynamic performance include: 1) maintaining the input currents so that they are in sinusoidal form and in phase with the input voltages so they have a unity power factor, 2) keeping the dc-link voltage within the reference amount, 3) keeping the output voltages at a fixed amount and 4) keeping the output voltages in sinusoidal and symmetrical forms. Simulation results indicate the potential and capability of the proposed method in improving DEPT behavior.

• Journal of Electrical Engineering and Technology
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• v.12 no.2
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• pp.738-750
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• 2017

In this paper, carrier-based pulse width modulation (CBPWM), space vector PWM (SVPWM) and reduced switching losses PWM (RSLPWM) for the three-level neutral point clamped (NPC) inverter are introduced. In the case of the neutral point (NP) potential (NPP) offset, an asymmetric disposition PWM (ASPDPWM) strategy is proposed, which can output PWM sequences correctly and suppress the lower order harmonics of the inverter effectively. An NPP balance strategy based on carrier based PWM (CBPWM) is analyzed. A hybrid modulation strategy combining RSLPWM and the NPP balance based on CBPWM is proposed, and hysteresis control is adopted to switch between the two modulation strategies. An experimental prototype of the three-level NPC inverter is built. The effectiveness of the hybrid modulation is verified with a resistance-inductance load and a permanent magnetic synchronous motor (PMSM) load, respectively. The experimental results show that reduced switching losses and an acceptable NPP can be effectively achieved in the hybrid modulation strategy.

• 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.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.23 no.9
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• pp.124-129
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• 2009

Recently, PWM inverter is widely utilized for many industrial applications such as high performance drive and space vector pulse width modulation(SVPWM) inverter which has high voltage ratio and low harmonics compared to conventional PWM inverter. This paper presents the implementation on a field programmable gate array(FPGA) of a SVPWM module for a voltage source inverter. The SVPWM module consists of PWM generator, current and position sensor interface and dead time compensator. The implemented SVPWM module can be integrated with a digital signal processor(DSP) to provide a flexible and effective solution for high performance voltage source inverter and for the use of multi-motor control. The performance of SVPWM module is verified by simulation and several experimental results.

• Proceedings of the KIEE Conference
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• 1998.07f
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• pp.2051-2053
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• 1998

In spite of nonlinear or step-changing load and line disturbances, the inverter for the UPS must provide the pure sinusoidal output voltage with low THD(Total Harmonics Distortion). This paper proposes an inverter controller for the UPS which has a good dynamic response characteristic and robustness for applying industrial world directly. The inverter output voltage is controlled instantaneously with a double regulation loop by a TMS320C31 Digital Signal Processor so that it has very good dynamic response for nonlinear or step-changing load and line disturbances. To improve the voltage utilization and response characteristics, the Space Vector Modulation(SVM) technique is adapted for the switching method of this system. The characteristics of the proposed control system were verified by simulations and experiments.

• Journal of Power Electronics
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• v.14 no.2
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• pp.392-401
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• 2014

A fast-transient repetitive control strategy for a three-phase shunt active power filter is presented in this study to improve dynamic performance without sacrificing steady-state accuracy. The proposed approach requires one-sixth of the fundamental period required by conventional repetitive control methods as the repetitive control time delay in the synchronous reference frames. Therefore, the proposed method allows the system to achieve a fast dynamic response, and the program occupies minimal storage space. A proportional-integral regulator is also added to the current control loop to eliminate arbitrary-order harmonics and ensure system stability under severe harmonic distortion conditions. The design process of the corrector in the fast-transient repetitive controller is also presented in detail. The LCL filter resonance problem is avoided by the appropriately designed corrector, which increases the margin of system stability and maintains the original compensation current tracking accuracy. Finally, experimental results are presented to verify the feasibility of the proposed strategy.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.10 no.4
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• pp.53-66
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• 1990

Rainfall is a phenomenon that shows a high variability both in space and time, Hy drologists are usually interested in the description of spatial distribution of rainfall over watershed. The theory of Kriging, generalized covariance technique using nonstationary mean in the regions under orographic effect, was chosen to construct random surface of total storm depth. For the constructed random surface, the double Fourier analysis of the total storm depths was performed, and the principal harmonics of storm were determined. The local component, or storm residuals was obtained by subtracting the periodic component of the storm from total storm depths. It is assumed that the residuals are a sample function of a homogeneous random field. This random field can be characterized by an isotropic one dimensional autocorrelation function or its corresponding spectral density function. Under this assumption, this study proposed a theorectical model for spectral density function adapted to two watersheds.

• Journal of Power Electronics
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• v.13 no.5
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• pp.806-813
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• 2013

This paper aims to compare the performance of three phase induction motor drives using Five Leg Inverter (FLI) and Three Leg Inverter (TLI) configurations. An Indirect Field Oriented Control (IFOC) method using a TLI is well established and incorporated for high performance speed drives in various industries. The FLI dual motor drive system on the other hand shows good workability in the independent control of two induction motor drives simultaneously. In this experiment, the IFOC method is utilized for both drive systems, and Space Vector Pulse Width Modulation (SVPWM) is used to generate pulses for both inverters. For the FLI, the Double Zero Sequence (DZS) Injection technique is used to generate the modulation signal. The complete experiment setup is done by using a DSpace 1103 controller board. The individual motor performances are analyzed using similar schemes, equipment setups and controller parameter values. The results show similar speed performance response capability between the single motor operation using a TLI system and the two motor operation using a FLI system based on the variable speed range either in forward or reverse operation. They also show similar load rejection abilities. However, the single motor with a TLI has a better power quality aspect such as ripple current and total harmonics distortion (THD).