• 대한전기학회논문지:전력기술부문A
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• 제53권9호
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• pp.500-506
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• 2004

This paper proposes a modified current differential relay for transformer protection unaffected by the remanent flux. The relay uses the same restraining current as a conventional relay, but the differential current is modified to compensate for the effects of the exciting current. To cope with the remanent flux, before saturation, the relay calculates the core-loss current and uses it to modify the measured differential current. When the core then enters saturation, the initial value of the flux is obtained by inserting the modified differential current at the start of saturation into the magnetization cure. Thereafter, the actual core flux is then derived and used in conjunction with the magnetization curve to calculate the magnetizing current. A modified differential current is then derived that compensates for the core-loss and magnetizing currents. The performance of the proposed differential relay was compared against a conventional differential relay. Results indicate that the modified relay remained stable during severe magnetic inrush and over-excitation because the exciting current was successfully compensated. This paper concludes by implementing the relay on a hardware platform based on a digital signal processor. The relay discriminates magnetic inrush and over-excitation from an internal fault and is not affected by the level of remanent flux.

• Journal of Electrical Engineering and Technology
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• 제13권4호
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• pp.1623-1630
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• 2018

Normally, various auxiliary loads are installed along with the main load in industrial applications. Usually, load transformers are used to convey such types of auxiliary loads. The transformers become energized when the loads are turned-on, consequently, high amplitude of inrush current appears at the output of the uninterrupted power supply (UPS) system. To mitigate these high current amplitudes, this manuscript suggests a three-phase line-interactive UPS system to counter the inrush current during the turning-on of the auxiliary load transformer while powering the main load by using a current controlled inverter. Experimental results of a laboratory-sized prototype are provided in the support of the proposed UPS system for validation.

• Journal of Electrical Engineering and Technology
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• 제9권3호
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• pp.1104-1111
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• 2014

The digital substations are being built based on the IEC 61850 network. The cooperation and protection of power system are becoming more intelligent and reliable in the environment of digital substation. This paper proposes a novel method to prevent the malfunction caused by the Transformer Magnetizing Inrush Current(TMIC) using the IEC 61850 based data sharing between the IEDs. To protect a main transformer, the current differential protection(87T) and over-current protection(50/51) are used generally. The 87T IED applies to the second harmonic blocking method to prevent the malfunction caused by the TMIC. However, the 50/51 IED may malfunction caused by the TMIC. To solve that problem, the proposed method uses a GOOSE inter-lock signal between two IEDs. The 87T IED transmits a blocking GOOSE signal to the 50/51 IED, when the TMIC is detected. The proposed method can make a cooperation of digital substation protection system more intelligent. To verify the performance of proposed method, this paper performs the real time test using the RTDS (Real Time Digital Simulator) test-bed. Using the RTDS, the power system transients are simulated, and the TMIC is generated. The performance of proposed method is verified in real-time using that actual current signals. The reaction of simulated power system responding to the operation of IEDs can be also confirmed.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2001년도 하계학술대회 논문집 A
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• pp.485-487
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• 2001

Current differential protective relaying with harmonic restraint module is, in general, used to protect transformer. But It is hard to distinguish inrush and internal winding fault with differential current protective relaying. This paper presents the new protective algorithm for transformers using 3-phase differential current's d1 coefficient values in wavelet transform. Various states of transformer was simulated using EMTP. Internal winding fault and inrush are classified within shorter time using the proposed algorithm.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2008년도 제39회 하계학술대회
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• pp.85-86
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• 2008

This paper analyzes the improvement of the power quality by the reduction of the inrush current. We analyze the existing methods and simulate the segragated point-on-wave closing method selected as the proper method. And we attempts to use the resistive type Superconducting Fault Current Limiter(SFCL) to reduce the transformer inrush current. We simulate the various insertion resistances and analyze the voltage drop. All simulation are performed by EMTP. The simulation results show the validity and effectiveness of a SFCL application and the segragated point-on-wave closing method to reduce the inrush current and improve the power quality.

• 대한전기학회논문지:전력기술부문A
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• 제52권12호
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• pp.691-697
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• 2003

This paper proposes a current differential relay for transformer protection that operates in accordance with a blocking method based on the difference-function of a differential current. For magnetic inrush and over-excitation, discontinuities in the first-difference function of the differential current arise at the points of inflection, which correspond to the start and end of each saturation period of the core. These discontinuities are converted into the pulses in the second- and third-difference functions of the differential current. The magnitudes of the pulses are large enough to detect saturation of the core. A blocking signal is issued if the magnitude of the third-difference function exceeds the threshold and is maintained for three quarters of a cycle. The performance of the relay is assessed under various conditions with magnetic inrush, internal faults and external faults. The proposed blocking method can improve significantly the operating time of a relay and achieve high sensitivity of a relay.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2006년도 추계학술대회 논문집 전력기술부문
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• pp.363-365
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• 2006

This paper proposes an estimation method for a circulating current of a Y-${\Delta}$ Transformer. The delta winding current can be decomposed into the two components i.e. a non-circulating component and a circulating component. The former can be estimated using the line currents. However, the latter can not be estimated directly using the line currents. A first order differential equation for the circulating current is derived by applying the Kirchhoff's voltage law on the loop of the delta side. The circulating current can be estimated by the solving the differential equation. The performance of the proposed algorithm is investigated under various conditions including magnetic inrush and over-excitation. The algorithm can estimate the circulating current very accurately even under magnetic inrush and over-excitation.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2003년도 추계학술대회 논문집 전력기술부문
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• pp.262-265
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• 2003

During magnetic inrush or over-excitation saturation of the core in a transformer draws a large exciting current. This can cause mal-operation of a differential relay. This paper proposes a modified current differential relay for transformer protection. In order to cope with the remanent flux at the beginning. the start of saturation of the core is detected and the core flux at the instant is estimated by inserting the differential current into a magnetization curve. Then, this core flux value can be used to calculate the core flux. The proposed relay calculates the 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 current and the magnetizing current from the conventional differential current. The proposed technique not only discriminates magnetic inrush and over-excitation from an internal fault, but also improves the speed of the conventional relay.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1997년도 하계학술대회 논문집 D
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• pp.920-923
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• 1997

This paper describes the analysis of characteristics of inrush current in transformers using wavelet transform. The simulations are performed with various transformer conditions, including transformer switching-on angles and the remanent flux. The ratio between wavelet coeffients is used for analysis.

• 전기학회논문지
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• 제67권1호
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• pp.29-37
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• 2018

Inrush current and fault current in a transformer need to be distinguished from one another. In order to do this, KEPCO uses a 2nd harmonic restraint/block method. We use two setting values for 2nd harmonic restraint; 15% and 10%. We also apply per-phase blocking method among various harmonic restraint methods. If the transformer is located at the radial system, we adjust 10% in the 2nd harmonic restraint, but this method is not enough to prevent mal-operations of the current differential relay and let us spend more time to change setting value again as the power system changes. In this paper, a more reasonable setting value for a 2nd harmonic blocking scheme in KEPCO is proposed. To present a proposed method, the fault data of the current differential relays which have occurred since 2009 are analyzed. To evaluate the performance of the proposed method, the results of the RTDS test for the current differential relay of the transformer by KEPCO are analyzed.