• The Transactions of the Korean Institute of Electrical Engineers A
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• v.53 no.2
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• pp.80-86
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• 2004

During magnetic inrush or over-excitation, saturation of the core in a transformer draws a large exciting current, which can cause mal-operation of a differential relay. This paper proposes a modified current differential relay for transformer protection. The relay calculates core-loss current from the induced voltage and the core-loss resistance; the relay calculates the magnetizing current from the core flux and the magnetization curve. Finally, the relay obtains the modified differential current by subtracting the core-loss and the magnetizing currents from the conventional differential current. Comparison study with the conventional differential relay with harmonic blocking is also shown. The proposed technique not only discriminates magnetic inrush and over-excitation from an internal fault, but also improves the speed of the conventional relay.

• KIEE International Transactions on Power Engineering
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• v.2A no.4
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• pp.153-159
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• 2002

Power transformer protective relay should block the tripping during magnetizing inrush and rapidly operate the tripping during internal faults. Recently, the frequency environment of power system has been made more complicated and the quantity of 2nd frequency component in inrush state has been decreased because of the improvement of core steel. And then, traditional approaches will likely be maloperate in the case of magnetizing inrush with low second harmonic component and internal faults with high second harmonic component. This paper proposes a new relaying algorithm to enhance the fault detection sensitivities of conventional techniques by using a fuzzy logic approach. The proposed fuzzy based relaying algorithm consists of flux-differential current derivative curve, harmonic restraint, and percentage differential characteristic curve. The proposed relaying was tested with relaying signals obtained from EMTP simulation package and showed a fast and accurate trip operation.

• KIEE International Transactions on Power Engineering
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• v.4A no.3
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• pp.146-151
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• 2004

The most widely used primary protection for the internal fault detection of the power transformer is current ratio differential relaying (CRDR) with harmonic restraint. However, the second harmonic component could be decreased by magnetizing inrush when there have been changes to the material of the iron core or its design methodology. The higher the capacitance of the high voltage status and underground distribution, the more the differential current includes the second harmonic during the occurrence of an internal fault. Therefore, the conventional second harmonic restraint CRDR must be modified. This paper proposes a numerical algorithm for enhanced power transformer protection. This algorithm enables a clear distinction regarding internal faults as well as magnetizing inrush and steady state. It does this by analyzing the RMS fluctuation of terminal voltage, instantaneous value of the differential current, RMS changes, harmonic component analysis of differential current, and analysis of flux-differential slope characteristics. Based on the results of testing with WatATP99 simulation data, the proposed algorithm demonstrated more rapid and reliable performance.

• The Transactions of the Korean Institute of Power Electronics
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• v.21 no.1
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• pp.10-18
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• 2016

In this study, an efficient clamp is proposed to reduce the switch voltage stress of a forward converter. The proposed clamp consists of a conventional LC snubber, a tertiary winding, and a diode. When the switch is turned OFF, the magnetizing inductor energy of the transformer is recovered directly into the flyback output, which is the tertiary winding and diode network, instead of circulating in the LC snubber. Therefore, switch voltage stress and circulating current caused by the magnetizing inductor energy are reduced. This condition improves the efficiency of the forward converter with limited switch voltage stress. A theoretical analysis and the design guidelines of the proposed converter are provided. Experimental results are also reported.

• KIEE International Transactions on Power Engineering
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• v.5A no.1
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• pp.1-8
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• 2005

During magnetic inrush or over-excitation, saturation of the core in a transformer draws a significant exciting current, which can cause malfunction of a current-differential relay. This paper proposes a modified-current-differential relay for transformer protection. The relay calculates the core-loss current from the induced voltage and the core-loss resistance as well as the magnetizing current from the core flux and the magnetization curve. Finally, the relay obtains the modified differential current by subtracting the core-loss and the magnetizing currents from the conventional differential current. A comparative study of the conventional differential relay with harmonic blocking is presented. The proposed relay not only discriminates magnetic inrush and over-excitation from an internal fault, but also improves the relay speed.

• The Transactions of the Korean Institute of Power Electronics
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• v.1 no.1
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• pp.27-37
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• 1996

This paper proposes the control algorithm for maximum efficiency drive of PWM inverter - induction motor system with high dynamic performance. If the induction motor is driven under light load with rated magnetizing current, the Iron loss is excessively large compared with the codder loss which results in doer motor efficiency. Maximum efficiency drive of an induction motor can be achieved by controlling the magnetizing current to satisfy the optimal ratio that leads the total motor loss to be a minimum value at a given speed. The proposed control algorithm essentially uses vector control technique and adopts voltage decoupling control strategy to prevent the degradation of dynamic performance due to reduced magnetizing current. To verify the proposed method, digital simulations and experiments are carried out for a squirrel-cage induction motor with the rating of 2.2[kW].

• Proceedings of the KIEE Conference
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• 2002.07b
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• pp.776-779
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• 2002

This paper describes the magnetizer design of highly saturated magnetic circuit by high external field. The magnetizer is loaded by a capacitor bank with impulse currents to reach high magnetic flux-density(typically 5[T]). Magnetizing current come out from magnetizing system is simulated by analytic method and transient FEM, also field distributions of the magnetizer are analyzed.

• Proceedings of the KIPE Conference
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• 1996.06a
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• pp.35-38
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• 1996

A new high power factor converter using transformer magnetizing energy for power factor correction with a single-switch/single-stage is proposed. The proposed converter gives the good power factor correction, low current harmonic distortions, and tight output voltage regulation. The prototype shows the IEC555-2 requirements are met satisfactorily with nearly unity power factor.

• Journal of Electrical Engineering and Technology
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• v.12 no.6
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• pp.2262-2267
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• 2017

A new force expression on dielectrics subjected to electric field is proposed in this paper. It is the electric version of the equivalent magnetizing current method in magnetic field. From the idea of electromagnetic duality, virtual equivalent electrifying magnetic current method is conjectured in the field of dielectric force problem. Numerical results show that the proposed method has good agreements with the conventional methods. The merits and demerits of the proposed method are also discussed.

• Journal of Electrical Engineering and Technology
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• v.1 no.3
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• pp.351-358
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• 2006

In this paper, the electrical characteristics of the contactless transformer is presented using the conventional coupled inductor theory. Compared with the conventional transformer, the contactless transformer has a large airgap, long primary wire and multi-secondary wire. As such, the contactless transformer has a large leakage inductance, small magnetizing inductance and poor coupling coefficient. Therefore, large magnetizing currents flow through the entire primary system due to small magnetizing inductance, resulting in low overall system efficiency. In high power applications, the contactless transformer is so bulky and heavy that it needs to be split by some light and small transformers. So, the contactless transformer needs several small transformer modules that are connected in series or parallel to transfer the primary power to the secondary one. This paper shows the analysis and measurement results of each contactless transformer module and comparison results between the series- and parallel-connection of the contactless transformer. The results are verified on the simulation based on the theoretical analysis and the 30kW experimental prototype.