• The Transactions of The Korean Institute of Electrical Engineers
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• v.67 no.12
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• pp.1705-1710
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• 2018

This paper deals with an analysis of the causes of over current relay(OCR) misoperation in power system with distributed generations(DG). In general, Y-D and Y-Y-D transformer connections are used for grid interconnection of DG. According to the interconnection guideline, the neutral point on Y side should be grounded. However, these transformer connections can lead to OCR misoperation as well as over current ground relay(OCGR) misoperation. Several researches have addressed the OCGR misoperation due to the interaction between transformer connections and zero-sequence voltage of distribution system. Recently, a misoperation of OCR at the point of DG interconnection to the utility system has been also reported. With increasing the interconnections of DG, such OCR as well as OCGR misoperations are expected to increase. In this paper, PSCAD/EMTDC modeling including DG interconnection transformer was performed and various case studies was carried out for identifying the cause of OCR misoperation.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.56 no.10
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• pp.1744-1750
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• 2007

This paper addresses grid interconnection tests of a 3kW transformer-less photovoltaic (PV) inverter to verify the effectiveness of the PV inverter and promote its wide use. The 3kW transformer-less PV inverter, which was equipped with intelligent protection and control system, was manufactured. A hardware test bed was constructed for performance tests of the PV inverter. Control performance and grid protection tests were carried out using the test bed. Test results verify the performance of the power control and grid protection functions of the developed transformer-less PV inverter.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.64 no.11
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• pp.1543-1550
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• 2015

Ferroresonance is a non-linear vibrational phenomenon that is generated by the electrical interaction of the inductance component with the capacitor component of a certain capacitance as the device of the inductance component such as a transformer is saturated due to the degradation, the waveform distortion of current and voltage, and the oscillation of overcurrent and overvoltage in a system. Recently, ferroresonance was generated from the waveform distortion of current and voltage, or the overvoltage or undervoltage phenomenon caused by the nature of an electrical power system and design technology of the transformer in the three phase transformer system. Hence, in general, ferroresonance analyzed by converting to the LC equivalent circuit. However, in general, the aforementioned analytical method only applies to the resonance phenomenon that is generated by the interaction of the capacitance of bussbar and grounding, and switching as the capacitor component with PT and the transformer as the inductance component in a system. Subsequently, the condition where ferroresonance was generated since overvoltage was supplied as line voltage to the phase voltage and thus the iron core is saturated due to the interconnection between grounded and ungrounded systems could not be analyzed when single phase PT was connected in a ${\Delta}$/Y connection system. In this study, voltage swell in the configuration of grounded circuit of a step-up transformer with the ${\Delta}-{\Delta}$ connection linked to PT for control power and the ferroresonance generated by overvoltage when the line voltage of the ${\Delta}-{\Delta}$ connection was connected to the phase voltage of the grounded Y-Y connection were analyzed using PSCAD / EMTDC through the failure case of the transformer caused by ferroresonance in the system with the ${\Delta}-{\Delta}$/Y-Y connection, and subsequently, the preventive measure of ferroresonance was proposed.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.58 no.10
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• pp.1916-1922
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• 2009

Introducing distributed generators(DGs) to utility distribution system can cause malfunction of relay on the grid when ground faults or severe load unbalances are occurred on the system. Because DGs interconnected to the grid can contribute fault currents and make bidirectional power flows on the system, fault currents from DGs can cause an interference of relay operation. A directional over current relay(DOCR) can determine the direction of power flow whether a fault occurs at the source side or load side through detecting the phases of voltage and current simultaneously. However, it is identified in this paper that the contributed fault current(Ifdg) from the ground source when was occurred to contribute single-line-to-ground(SLG) fault current, has various phases according to the distances from the ground source. It means that the directionality of Ifdg may not be determined by simply detecting the phases of voltage and current in some fault conditions. The magnitude of Ifdg can be estimated approximately as high as 3 times of a phase current and its maximum is up to 2,000 A depending on the capacity of generation facilities. In order to prevent malfunction of relay and damage of DG facilities from the contribution of ground fault currents, Ifdg should be limited within a proper range. Installation of neutral ground reactor (NGR) at a primary neutral of interconnection transformer was suggested in the paper. Capacity of the proposed NGR can be adjusted easily by controlling taps of the NGR. An algorithm for unidirectional relay was also proposed to prevent the malfunction of relay due to the fault current, Ifdg. By the algorithm, it is possible to determine the directionality of fault from measuring only the magnitude of fault current. It also implies that the directionality of fault can be detected by unidirectional relay without replacement of relay with the bidirectional relay.

• Journal of the Korea Convergence Society
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• v.2 no.4
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• pp.29-37
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• 2011

Dispersed generation (DG) such as wind power (WP) and Photovoltaic systems (PV) that has been promoted at the national level recently is mainly being introduced into distribution systems adjacent to consumers because it is generation on a small scale when compared to current generation. Due to its characteristics, DG can be operated by interconnection with distribution systems to present security of more stable power and efficient use of power facilities and resources. Problems on protection coordination of distribution systems by reverse flow of DG can roughly be divided into three possibilities: excess in rated breaking capacity (12.5KA) of protective devices by a fault in DG current supply, failure to operate protective devices by an apparent effect that can occur by reduction in impedance parallel circuit fault current due to interconnection of DG, and malfunction of protective devices by interconnection transformer connection type. The purpose of this study is to analyze problems in protection coordination that can occur when DG is operated by interconnection with distribution systems by conducting modeling and simulations by using theoretical symmetrical components and MATLAB/SIMULINK to present methods to improve such problems.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.62 no.11
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• pp.1604-1609
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• 2013

When the ground fault on the power side occurs on distribution system interconnected with distributed generations, the abnormal current is generated in the neutral conductor by the connection type and the iron core structure of transformers for the interconnection of distributed power supplies due to the unbalanced voltage of the system, and subsequently the false operation of the protective relay on the load side occurs. Herein, this paper proposes the method to correct errors in the phase current to prevent the false operation of the protective relay by applying p-q theory and presents the simulation result of the error correction algorithm using PSCAD/EMTDC.

• Korean Journal of Artificial Intelligence
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• v.11 no.4
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• pp.9-13
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• 2023

In this study, we explored the potential of integrating interactive AI callbot technology into the medical consultation domain as part of a broader service development initiative. Aimed at enhancing patient satisfaction, the AI callbot was designed to efficiently address queries from hospitals' primary users, especially the elderly and those using phone services. By incorporating an AI-driven callbot into the hospital's customer service center, routine tasks such as appointment modifications and cancellations were efficiently managed by the AI Callbot Agent. On the other hand, tasks requiring more detailed attention or specialization were addressed by Human Agents, ensuring a balanced and collaborative approach. The deep learning model for voice recognition for this study was based on the Transformer model and fine-tuned to fit the medical field using a pre-trained model. Existing recording files were converted into learning data to perform SSL(self-supervised learning) Model was implemented. The ANN (Artificial neural network) neural network model was used to analyze voice signals and interpret them as text, and after actual application, the intent was enriched through reinforcement learning to continuously improve accuracy. In the case of TTS(Text To Speech), the Transformer model was applied to Text Analysis, Acoustic model, and Vocoder, and Google's Natural Language API was applied to recognize intent. As the research progresses, there are challenges to solve, such as interconnection issues between various EMR providers, problems with doctor's time slots, problems with two or more hospital appointments, and problems with patient use. However, there are specialized problems that are easy to make reservations. Implementation of the callbot service in hospitals appears to be applicable immediately.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.58 no.8
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• pp.1512-1518
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• 2009

Recently, new dispersed power sources such as photovoltaics, wind power, fuel cell etc. are energetically interconnected and operated in the distribution systems, as one of the national projects for alternative energy. This paper deals with the optimal countermeasures for the mal-function of protective devices at primary feeder in distribution systems when new power sources like photovoltaic (PV) systems are interconnected, based on the symmetrical components of short circuit studies. When new power sources are considered to be interconnected to distribution systems, bi-directional power flow and interconnection transformer connection of new power sources may cause the operation problems of protective devices (mainly re-closer), since new power sources can change typical characteristics of distribution systems. Therefore, this paper shows an analysis skill of the mal-functional mechanism of protective relay and proposes the optimal solution for the mal-function problem using the symmetrical components of fault analysis. And, this paper also shows the effectiveness of proposed method by the simulation at the field distribution systems.

• Journal of Electrical Engineering and Technology
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• v.10 no.6
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• pp.2256-2261
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• 2015

In the power plant using high temperature fuel cells such as Molten Carbonate Fuel Cell(MCFC), and Solid Oxide Fuel Cell(SOFC), the generated electric power per area of power generation facilities is much higher than any other renewable energy sources. - High temperature fuel cell systems are capable of operating at MW rated power output. - It also has a feature that is short for length of the line for connecting the interior of the generation facilities. In normal condition, these points are advantages for voltage drops or power losses. However, in abnormal condition such as fault occurrence in electrical system, the fault currents are increased, because of the small impedance of the short length of power cable. Commonly, to minimize the thermal-mechanical stresses on the stack and increase the systems reliability, we divided the power plant configuration to several banks for parallel operation. However, when a fault occurs in the parallel operation system of power main transformer, the fault currents might exceed the interruption capacity of protective devices. In fact, although the internal voltage level of the fuel cell power plant is the voltage level of distribution systems, we should install the circuit breakers for transmission systems due to fault current. To resolve these problems, the SFCL has been studied as one of the noticeable devices. Therefore, we analyzed the effect of application of the SFCL on bus tie in a fuel cell power plants system using PSCAD/EMTDC.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.4
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• pp.2737-2743
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• 2015

The wind power plant must be able to produce reactive power at the POI bus of a wind farm connected to power system to keep or control the voltage of POI bus. But, the reactive power capability of wind turbines may not be sufficient to control the voltage of POI bus due to the reactive power losses in connection lines between wind farm and POI bus. The solution of this problem is to install an external STATCOM. The proposed cooperative control method of STATCOM and conventional reactive power compensators such as Switched-shunt and tap changing transformer can control the voltage of POI bus more efficiently. The simulation results are shown that the voltage drop of POI Bus of Test System with the arbitrary load change rate to initial loads is improved more than 60% and the voltage of load bus is maintained more than 95% of rated voltage.