• Journal of Electrical Engineering and Technology
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• v.9 no.5
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• pp.1471-1481
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• 2014

This paper presents the new power dissipation model of individual switching device in a high-level modular multilevel converter (MMC), which can be mostly used in voltage sourced converter (VSC) based high-voltage direct current (HVDC) system and flexible AC transmission system (FACTS). Also, the voltage balancing method based on sorting algorithm is newly proposed to advance the MMC functionalities by effectively adjusting switching variations of the sub-module (SM). The proposed power dissipation model does not fully calculate the average power dissipation for numerous switching devices in an arm module. Instead, it estimates the power dissipation of every switching element based on the inherent operational principle of SM in MMC. In other words, the power dissipation is computed in every single switching event by using the polynomial curve fitting model with minimum computational efforts and high accuracy, which are required to manage the large number of SMs. After estimating the value of power dissipation, the thermal condition of every switching element is considered in the case of external disturbance. Then, the arm modeling for high-level MMC and its control scheme is implemented with the electromagnetic transient simulation program. Finally, the case study for applying to the MMC based HVDC system is carried out to select the appropriate insulated-gate bipolar transistor (IGBT) module in a steady-state, as well as to estimate the proper thermal condition of every switching element in a transient state.

• Journal of Power Electronics
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• v.18 no.2
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• pp.547-557
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• 2018

This work proposes a control method of frequency stabilization for grid integration of large-scale wind farms via the voltage source converter-based high-voltage direct current (VSC-HVDC) technology. First, the topology of grid integration of a large-scale wind farm via the VSC-HVDC link is provided, and simple control strategies for wind turbines, wind farm side VSC (WFVSC), and grid side VSC are presented. Second, a mathematical model between the phase angle of WFVSC and the frequency of the wind farm is established. The control principle of the large-scale wind power integrated system is analyzed in theory in accordance with the mathematical model. Third, frequency and AC voltage controllers of WFVSC are designed based on the mathematical model of the relationships between the phase angle of WFVSC and the frequency of the wind farm, and between the modulation index of WFVSC and the voltage of the wind farm. Corresponding controller structures are established by deriving a transfer function, and an optimization method for selecting the parameters of the frequency controller is presented. Finally, a case study is performed under different operating conditions by using the DIgSILENT/PowerFactory software. Results show that the proposed control method has good performance in the frequency stabilization of the large-scale wind power integrated system via the VSC-HVDC technology.

• Journal of IKEEE
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• v.23 no.4
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• pp.1208-1217
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• 2019

The life-cycle prediction of the sub-module which is the unit system of MMC is very important from the viewpoint of maintenance and economic feasibility of HVDC system. However, the life-cycle prediction that considers only the type, number and combination of parts is a generalized result that does not take into account the operating condition of the sub-module, and may significantly differ from the life-cycle of the actual one. Therefore, we design a fault tree for the purpose of reflecting the operation characteristics of the full-bridge sub-module and apply the MIL-HDBK-217F to the failure rate of the basic event to predict the life-cycle of the full-bridge sub-module. It compares the life-cycle expectancy of the conventional failure rate analysis with the proposed fault-tree analysis and compares the lifetime according to whether the redundancy of the full-bridge sub-module is considered.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.63 no.11
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• pp.1491-1496
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• 2014

VSC-HVDC system is vulnerable to a DC fault because the fault current can be injected from AC system to DC system during the fault. Therefore, DC circuit breaker is required to isolate faults in VSC-HVDC system. The inverse current injection method of circuit breaker has been considered as DC circuit breaker. However, the topology has drawback that the breaking time is longer than hybrid circuit breaker using semiconductor devices. In order to solve this problem, this paper proposes an improved topology of circuit breaker based on inverse current injection method. In addition, the proposed topology will be compared with the existing topology. And we will verify its effects by using the simulation results.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.62 no.9
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• pp.1240-1248
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• 2013

This paper describes a switching-level operation analysis of BTB(Back-To-Back) converter for HVDC(high voltage DC) application based on MMC(modular multi-level converter). A switching-level operation analysis for BTB converter is very important to understand the converter operation in detail and check the voltage and current transients in each components. However, the development of switching-level simulation model for the actual size BTB Converter is very difficult because the MMC normally has more than 150 sub-modules for each arm. So, a switching level simulation model for the 11-level MMC-based BTB converter was developed with PSCAD/EMTDC software, which has 12 sub-modules for the positive arm and another 12 sub-modules for the negative arm. The DC-voltage balance algorithm, the circulating-current reduction algorithm, the harmonic reduction algorithm, and the redundancy operation algorithm were included in this simulation model. The developed simulation model can be utilized to analyze the MMC-based BTB converter for HVDC application in switching level and to develop the protection scheme for the MMC-based BTB converter for HVDC application.

• Journal of Electrical Engineering and Technology
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• v.10 no.2
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• pp.443-451
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• 2015

The stability of a multi-terminal direct current (MTDC) system is often influenced by its control strategy. To improve the stability of the MTDC system, the control strategy of the MTDC system must be appropriately adopted. This paper deals with estimating stability of a MTDC system based on the line-commutated converter based high voltage direct current (LCC HVDC) system with an inverter with constant extinction angle (CEA) control or a rectifier with constant ignition angle (CIA) control. In order to evaluate effects of two control strategies on stability, a MTDC system is tested on two conditions: initialization and changing DC power transfer. In order to compare the stability effects of the MTDC system according to each control strategy, a mathematical MTDC model is analyzed in frequency domain and time domain. In addition, Bode stability criterion and transient response are carried out to estimate its stability.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.65 no.9
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• pp.1479-1485
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• 2016

This paper presents DC voltage recovery time improvement method in DC link of High Voltage Direct Current (HVDC) with offshore wind farm. The wind farm should be satisfied Low Voltage Ride Through(LVRT) control strategy when grid faults occur. The LVRT control strategy indicates actions which have to be executed according to the voltage dip ratio and the fault duration. However, The LVRT control strategy makes between wind farm and power system through DC Link voltage when grid fault occurs. The de-loading scheme is one of the method to control the DC voltage. But de-loading scheme need to long DC voltage recovery time. Thus, this paper proposes an improved de-loading scheme and we analysis DC voltage and active power reference through a simulation.

• Proceedings of the KIEE Conference
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• 2009.07a
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• pp.357_358
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• 2009

According to the 4th BPE(Basic Plan for long-term Electricity Supply and Demand), Electricity demand of Jeju island in 2012 will be reached to 682MW. Jeju island will be faced with severe shortage of electricty, also needed to find transport route to supply green energy which will be made from "Carbon free island" energy policy toward land at that time. For that reason, The plan which construct 400MW size HVDC connection line was decided for potential supply stabilization and transportation of green energy. KEPCO organized special construction office not only to observe the successful project completion which aims 2011 December but also to build up professional manpower. KEPCO is putting spur to the HVDC project these days. Construction site of converter stations has been already confirmed and contracts of cable, converter have done as per turn-key early this year. On this report, we would like to discuss about project scheme, main characteristics of system, and the furture progress plan of "Jindo~Jeju HVDC construction project expecially converter station part" which is being pushed by KEPCO.

• Proceedings of the KIEE Conference
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• summer
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• pp.597-601
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• 2004

HVDC(High Voltage Direct Current) is an underwater cable between Jeju Island and Haenam in main land and supplies approximately $50\%$ of electrical usage in Jeju Island. If there is any power failure due to HVDC, it will cost approximately 50,000 US dollars per day including Thermal Electrical Generation. Therefore it is absolutely necessary to recover the problem in rapid timely basis. Present survey method in Korea is done by scuba diver with air cylinder resulting very poor visual inspection. Other option is by only visual camera attached on miniature ROV for solely suey Purpose. This method does not includeburial depth of cable, cable position, cable condition & etc??‥‥.??? In result, current method does not generate any scientific or sophisticated data which does not allow any intelligent management decision. In conclusion, new method and new systems are needed urgently to upgrade current HVDC underwater cable survey technique in Korea to minimize the cost and time factors.

• Proceedings of the KIEE Conference
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• 2007.11b
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• pp.210-213
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• 2007

오늘날 풍력발전은 신재생에너지 중에서 실효성 및 경제성이 우수한 에너지원으로 지속적인 기술개발에 따라 규모의 대형화와 더불어 이의 확산이 빠르게 진행되고 있어 전력계통에서 차지하는 비중이 점점 증대되고 있다. 바람이 많은 제주도는 풍력발전에서 비교적 유리한 지역으로 평가되고 있어 앞으로 많은 풍력발전설비가 설치될 전망이다. 그러나 풍력발전은 특성상 전력품질 적정 유지 및 전력공급 신뢰도 측면에서 취약한 형태이며 특히 HVDC로 내륙 전력계통과 연계된 제주 전력계통은 HVDC 운전 형태에 따라 순동예비력 확보가 다르게 되어 계통주파수 제어에 어려움이 있게 된다. 본 논문은 HVDC로 연계된 소규모 전력계통인 제주 전력계통의 풍력발전 중대에 따른 계통주파수 변화 특성을 과도안정도 해석프로그램으로 분석하여 풍력발전시스템 계통 연계관련 안전성 등을 조사한 것이다.