• International Journal of Control, Automation, and Systems
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• v.4 no.3
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• pp.365-371
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• 2006

This paper presents an algorithm based on a combination of Discrete Wavelet Transforms and neural networks for detection and classification of internal faults in a two-winding three-phase transformer. Fault conditions of the transformer are simulated using ATP/EMTP in order to obtain current signals. The training process for the neural network and fault diagnosis decision are implemented using toolboxes on MATLAB/Simulink. Various cases and fault types based on Thailand electricity transmission and distribution systems are studied to verify the validity of the algorithm. It is found that the proposed method gives a satisfactory accuracy, and will be particularly useful in a development of a modern differential relay for a transformer protection scheme.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.63 no.3
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• pp.414-417
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• 2014

At present, the demand for electric power increases, the electric power system is complicated. The size of the line-to-ground fault and the line-to-line fault occurred with complication of electric power system continue to increase, therefore several issues are raised. To address these issues effectively, the superconducting fault current limiter (SFCL) has been proposed, this study is ongoing. In this paper, we applied the SFCL in three-phase transformer and comparative analysis of the electric power burden to the SFCL. The superconductor is combined to the third winding of transformers in connection structure. In case of a third line-to-line fault, we did comparative analysis of the electric power burden to the SFCL based on the turn ratio of transformer third winding. In this case, we could confirm as the third turn ratio increased, electric power impressed to the superconducting element increased.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2001.07a
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• pp.149-152
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• 2001

A great deal of attention has been focused on the application of miniaturized piezoelectric transformers to the low power source for the microsystem. The dielectric and piezoelectric properties of Pb(Mn,W,Sb,Nb)O$_3$-Pb(Zr,Ti)O$_3$ ceramics have been investigated on different calcination(750$^{\circ}C$∼950$^{\circ}C$) and sintering(1100$^{\circ}C$∼1300$^{\circ}C$) temperatures. The perovskitic phase was formed by the solid phase reaction of the oxides. Anisotropic (k$\sub$t/k$\sub$p/) properties of electromechanical coupling coefficient and piezoelectric coefficient have been proven to be depending on processing temperatures. The value of electromechanical coupling factor of K$\sub$p/>0.51 and a mechanical quality factor of Q$\sub$m/>2000 were obtained. The piezoelectric transformer was prepared using this ceramics with the composition of Pb(Mn,W,Sb,Nb)O$_3$-Pb(Zr,Ti)O$_3$ We studied the influence of different processing temperature on the microstructure and piezoelectric properties of complex PZT-based ceramics. and the characteristic of piezoelectric transformer.

• Proceedings of the KIEE Conference
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• summer
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• pp.607-609
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• 2004

A breakdown occurred in power transformer causes interruption of power transmission. Protective relay should be installed in transformer to detect such a fault. Protective relaying algorithm for transformer must be included a function to discriminate between winding fault and inrushing state. Recently, current differential relay is widely used to protect power transformer. However if inrush occurs in transformer, relay can be tripped by judging as internal fault. New algorithms are required in order to such problem. This study proposes a new protective relaying algorithm using Neuro-Fuzzy inference and wavelet. A variety of transformer transient states are simulated by BCTRAN and HYSDT in EMTP. D1 coefficients of differential current are obtained by wavelet transform. D1 coefficients and RMS of 3-phase primary voltage are used to make a target data and are trained by Nwo-Fuzzy algorithm which distinguishes correctly whether internal fault occurs or not within 1/2 after fault detection. It is evaluated that the results obtained by simulations can effectively protect a transformer by contact discriminating between winding fault and inrushing state.

• Proceedings of the KIEE Conference
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• 1996.07a
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• pp.388-391
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• 1996

The conventional high frequency phase-shifted full bridge DC-DC converter has a disadavantage that a circulating current flows through transformer and switching devices during the freewheeling interval Due to this circulating current, RMS current stress, conduction losses of transformer and switching devices are increased. To alleviate this problem, this study provides a novel circulating current free type high frequency soft switching phase-shifted full bridge DC-DC converter which applies the energy recovery snubber(ERS) attached at the secondary side of transformer. The ERS adopted in this study is consisted of three fast recovery diode($Ds_1$, $Ds_2$, $Ds_3$), two resonant capacitor($Cs_1$, $Cs_2$) and a small resonant inductor [(Lr) : It can be ignored because the transformer leakage inductance(Ll) is able to use in stead of inserting the resonant inductor(Lr)]

• Proceedings of the KIPE Conference
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• 2000.07a
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• pp.355-359
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• 2000

This paper deal with the harmonic reduction technique in three-phase square-wave inverter system formed by single-phase inverter. To reduce the harmonics six single-phase inverters are used for forming multi-phase inverter and zig-zag connected output transformer for eliminating the harmonic 6(2k-1)$\pm$1 orders. And an ac filter is furnished at output side. Computer simulations show that the THD of the output voltage can be reduced immensely.

• The Transactions of the Korean Institute of Power Electronics
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• v.13 no.3
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• pp.237-245
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• 2008

Recently, multilevel H-bridge inverters have become popular in medium to high power ac drive applications. One of significant advantages of them is low harmonic contents in their input line currents thanks to the transformer with multiple phase-shifted secondary windings. This paper attempts to provide basic guidelines for the design of the phase shifting transformer windings and theoretical analysis of input line current harmonics of H-bridge inverters. The part II is devoted to the analysis of the harmonic characteristics of the input line current, providing mathematical background for the equidistant phase-shifting angle distribution policy for harmonic elimination.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.63 no.3
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• pp.358-366
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• 2014

This paper proposes a quasi Z-source AC-AC converter with the low DC voltage distribution capability operating as a power electronic transformer. The proposed system has configuration that the input terminals of two quasi Z-source AC-AC converters are connected in parallel, also their output terminal are connected in series. Simple control method of duty ratio was proposed for the in phase buck-boost AC voltage mode and the DC output voltage control. DSP based experiment and PSIM simulation were performed. As a result, the PSIM simulation results were same with the measured results. By controlling the duty ratio under the condition of 100 [${\Omega}$] load, quasi Z-source AC-AC converter could buck and boost the AC output voltage in phase with the AC input voltage, and the same time, the constant DC voltage could be output without affecting the AC output characteristics. And, the DC output voltage 48[V] was constantly controlled in dynamic state in case while the load is suddenly changed ($50[\Omega]{\rightarrow}100[\Omega]$). From the above result, we could know that the quasi Z-source AC-AC converter can act as a power electronic transformer with a low DC voltage distribution capability.

• Korean Journal of Materials Research
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• v.15 no.12
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• pp.814-817
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• 2005

In order to enhance energy efficiency in high electric conversion devices such as Power transformers, which need to have high power properties, an alloy design approach in $Pb(Zr,Ti)O_3(PZT)$ base ceramic system was attempted $0.03Pb(Sb_{0.5}Nb_{0.5})O_3-0.03Pb(Mn_{1/3}Nb_{2/3})O_3-(0.94-x)PbTiO_3-xPbZrO_3$[PSN-PMN- PZT] ceramics were synthesized by conventional bulk ceramic processing technique. To improve power properties, the various Zr/Ti ratio was varied ]lear their morphotropic phase boundary (MPB) composition of PSN-PMN-PZT system and their effects on subsequent piezoelectric and dielectric properties for the transformer application at high power were systematically investigated using an impedance analyzer. Microstructure and phase information were characterized using X-ray diffractometer (XRD), a scanning electron microscope (SEM) and others. When the Zr/Ti ratio was 0.415/0.465, the value of $Q_m\;and\;k_p$ were shown to reach to the maximum, indicating that this alloy design can be a feasible composition :or high power transformer.

• Proceedings of the KIEE Conference
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• 2004.11a
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• pp.220-222
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• 2004

The piezoelectric ceramics for a step-up piezoelectric transformer are desired large electromechanical coupling factor, high mechanical quality factor and good characteristic resonance frequency. In this study, the empirical formula of specimens is used $0.9Pb(Zr_xTi_{1-x})O_3-0.1Pb(Mn_{1/3}Nb_{1/3}Sb_{1/3})O_3$ (PZT-PMNS). The piezoelectric and dielectric characteristic are investigated by sintering temperature and value of x as functions of $Ti^{2+}$, $Zi^{2+}$ mol rate. MPB(morphotropic phase boundary) is defined in the x=0.522. Because it is appeared to the best piezoelectric and dielectric characteristic in the x=0.522, it can be application of step-up transformer.