• 전기학회논문지
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• 제60권6호
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• pp.1128-1133
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• 2011

In a wind farm, a large number of small wind turbine generators (WTGs) operate whilst a small number of a large generator do in a conventional power plant. To maintain high quality and reliability of electrical energy, a wind farm should have equal performance to a thermal power plant in the transient state as well as in the steady state. The wind farm shows similar performance to the conventional power plant in the steady state due to the advanced control technologies. However, it shows quite different characteristics during fault conditions in a grid, which gives significant effects on the operation of a wind farm and the power system stability. This paper presents an analysis of response of a wind farm during grid fault conditions. During fault conditions, each WTG might produce different frequency components in the voltage. The different frequency components result in the non-fundamental frequencies in the voltage and the current of a wind farm, which is called by "beats". This phenomenon requires considerable changes of control technologies of a WTG to improve the characteristics in the transient state such as a fault ride-through requirement of a wind farm. Moreover, it may cause difficulties in protection relays of a wind farm. This paper analyzes the response of a wind farm for various fault conditions using a PSCAD/EMTDC simulator.

• Journal of Electrical Engineering and Technology
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• 제2권3호
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• pp.300-305
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• 2007

This paper reports an investigation of overvoltages caused by ferroresonance in the distribution system, which consists of a bank of open-delta single-phase voltage transformers. The high voltage sides of the voltage transformer are connected to the distribution system via three single-phase fuse cutouts. Due to the saturation characteristic of the transformer cores, ferroresonance can occur in the condition that the transformer is energized or deenergized with single-phase switching operation of the fuse cutouts. The simulation tool based on EMTP is used to investigate the overvoltages at the high side of voltage transformer. Bifurcation diagrams are used to present the ferroresonance behavior in ranges of different operating conditions. The simulation results are in good agreement with the results from the experiment of 22 kV voltage transformers. The mitigation technique with additional damping resistors in the secondary windings of the voltage transformers will be introduced. Brief discussion will be made on the physical phenomena related to the overvoltage and the damage of voltage transformer.

• Journal of Electrical Engineering and Technology
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• 제6권4호
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• pp.474-483
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• 2011

Solid-state fault current limiters (SSFCL) in power systems are alternative devices to limit prospective short circuit currents from reaching lower levels. Fault current limiters (FCL) can be classified into two categories: R-type (resistive) FCLs and L-type (inductive) FCLs. L-type FCL uses an inductor to limit fault level and is more efficient in suppressing voltage drop during a fault. In contrast, R-type FCL is constructed with a resistance and is more effective in consuming the acceleration energy of generators during a fault. Both functions enhance the transient stability of the power system. In the present paper, a novel SSFCL is proposed to enhance power system transient stability and power quality. The proposed SSFCL uses both functions of an L-type and R-type FCL. SSFCL consists of four diodes, one self-turn-off IGCT, a current-limiting by-pass inductor (L), and a variable resistance parallel with an inductor for improvement of power system stability and prevention of over-voltage across SSFCL. The main advantages of the proposed SSFCL are the simplicity of its structure and control, low steady-state impedance, fast response, and the existence of R-type and Ltype impedances during the fault, all of which improve power system stability and power quality. Simulations are accomplished in PSCAD/EMTDC.

• Journal of Electrical Engineering and Technology
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• 제6권2호
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• pp.255-262
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• 2011

This paper proposes a grid-tied power conditioning system for fuel cell, which consists of three-phase current-fed DC-DC converter and three-phase PWM inverter. The three-phase current-fed DC-DC converter boosts fuel cell voltage of 26-48 V up to 400 V with zero-voltage switching (ZVS) scheme, while the three-phase PWM(Pulse Width Modulation) inverter controls the active and reactive power supplied to the grid. The operation of the proposed power conditioning system with fuel cell model is verified through simulations with PSCAD/EMTDC software. The feasibility of hardware implementation is verified through experimental works with a laboratory prototype with 1.2 kW proton exchange membrane (PEM) fuel cell stack. The proposed power conditioning system can be commercialized to interconnect the fuel cell with the power grid.

• 전기학회논문지
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• 제57권11호
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• pp.1954-1961
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• 2008

Microgrid usually consists of a cluster of distributed generators(DGs), energy storage systems and loads, and can operate in the grid-connected mode and the islanded mode. This paper presents detailed descriptions of two different options for controlling the active power of DGs in the microgrid. One is regulating the power injected by the unit to a desired amount(Unit output power control) and the other is to regulate the flow of active power in the feeder where the unit is installed to a constant(Feeder flow control). Frequency-droop characteristics are used to achieve good active power sharing when the microgrid operates in the islanded mode. The change in the frequency and the active power output of DGs are investigated according to the control mode and the configuration of DGs when the microgrid is disconnected from the main grid. From the analysis, this paper proposes a method to determine the droop constant of DGs operating in the feeder flow control mode. Simulation results using the PSCAD/EMTDC are presented to validate the approach, which shows good performance as opposed to the conventional one.

• 전기학회논문지
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• 제62권7호
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• pp.925-930
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• 2013

The fault current of the power transmission system is greater than that of the power distribution system. Therefore, the introduction of superconducting fault current limiter (SFCL) is more needed to reduce the increased fault current. The trigger-type SFCL consists of the high-temperature superconducting element (HTSC), the current limiting reactor (CLR) and the circuit breaker (CB). The trigger-type SFCL can be used to supplement the disadvantages of the resistive-type SFCL. The operation characteristics of the current ratio differential relay which is usually applied to the protection device of the power transmission system are expected to be affected under fault conditions and the applicability of the trigger-type SFCL. In this paper, we analyzed the operating characteristics, by the fault conditions, between the current ratio differential relay for line protection and the trigger-type SFCL in the power transmission system through the PSCAD/EMTDC simulation.

• Journal of Electrical Engineering and Technology
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• 제12권6호
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• pp.2208-2218
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• 2017

To resolve overload in a distribution system, a distribution system operator (DSO) often performs a load transfer using normally open tie points and switches in the distribution line. During this process, the distribution system is momentarily operated in closed-loop operation. A closed-loop current in the distribution system can cause a power failure due to excess breaking current in the circuit breakers and reclosers. Therefore, it is necessary to calculate the closed-loop current exactly. However, if there are a large number of distributed generation (DG) systems in the distribution system, such as energy storage systems (ESS), they might obstruct the closed-loop operation based on bidirectional power flow. For quick and precise operation of a closed-loop system, the ESS has to regulate the power generation while satisfying closed-loop operation in the worst cases. We propose a strategy for balanced power regulation of an ESS. Simulations were carried out using PSCAD/EMTDC, and the results were compared with calculation results.

• 조명전기설비학회논문지
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• 제18권4호
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• pp.88-96
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• 2004

전압저하는 전력계통상의 사고에 의해 부하전압의 크기가 일시적으로 감소하는 현상으로서 주로 단락회로 사고에 의해 발생한다. 산업공정에서 하나의 장비가 전압저하에 의해서 문제를 일으킨다면 네트워크처럼 연결된 전체설비에 파급적인 영향을 미치기 때문에 그로 인한 경제적 손실은 상당히 크게 발생한다. 따라서 전압저하를 효과적으로 대처할 수 있는 방법이 전력품질 향상을 위해서 상당히 중요하게 대두되고 있다. 본 논문에서는 배전계통에서 단락사고로 발생한 전압저하 현상을 저감하고자 FCL(Fault Current Limiter)장치를 사용하여 전압저하 현상을 효과적으로 대처할 수 있는 방법을 제시하고, PSCAD/EMTDC 컴퓨터 시뮬레이션 프로그램을 이용하여 전압저하 저감효과를 분석하였다.

• Journal of Power Electronics
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• 제18권4호
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• pp.1111-1126
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• 2018

In modern power systems, distributed generators (DGs) result in high stress on system frequency stability. Apart from the intermittent nature of DGs, most DGs do not contribute inertia or damping to systems. As a result, a new control method referred to as a virtual synchronous machine (VSM) has been proposed, which brought new characteristics to inverters such as synchronous machines (SM). DGs employing an energy storage system (ESS) provide inertia and damping through VSM control. Meanwhile, energy storage presents some physical constraints in the VSM implementation level. In this paper, a VSM mathematical model is built and analyzed. The dynamic responses of the output active power are presented when a step change in the frequency occurs. The influences of the inertia constant, damping factor and operating point on the ESS volume margins are investigated. In addition, physical constraints are proposed based on these analyses. The proposed physical constraints are simulated using PSCAD/EMTDC software and tested through RTDS experiment. Both simulation and RTDS test results verify the analysis.

• 전기학회논문지
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• 제64권6호
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• pp.864-871
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• 2015

Recently, one of the main research on the power distribution system is the microgrid. The microgrid is a combination of power sources and loads, which is controllable and has separable connection. The main objective of microgrid is the deployment of the renewable clean energy and the enhancement of load-side reliability. The modern power sources and loads have DC I/O interfaces, which is the major advantage of DC microgrid compared to the conventional AC grid. The components in the microgrid have diverse features, so there is need of proper coordination control. For achieving economic feature, the active power of renewable energy resources is regarded as major control parameter and the whole operation modes of DC microgrid are defined, and the proper operations of each component are described. From the inherent characteristics of DC, there are two control variables: voltage and active power. Through analysis of operation modes, it is possible to determine exact control objectives and optimized voltage & power control strategy in each mode. Because of consideration of whole operation modes, regardless of the number and capacity of components, this coordination control method can be used without modification. This paper defines operation mode of DC microgrid with several DC sources and suggests economic and efficient coordinated control methods. Simulation with PSCAD proves effectiveness.