• Journal of IKEEE
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• v.24 no.1
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• pp.216-224
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• 2020

This paper proposes an impedance design method of the test device for evaluating Low Voltage Ride Through(LVRT) and High Voltage Ride Through(HVRT) functions. The LVRT/HVRT test device should have ability to generate the fault voltage specified in the grid code for a certain period and to limit the magnitude of the fault current with the design specification. In this paper, the impedance design method for auto transformer is proposed based on a equivalent model of a tap-change auto-transformer during LVRT/HVRT operation. In addition, to generate various fault voltages required the LVRT/HVRT test, tap impedance design in the auto transformer is considered. To verify the validity of the proposed design method, the design process of the 10MVA LVRT/HVRT test device was conducted and the design results was verified through simulation models.

• The Transactions of the Korean Institute of Power Electronics
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• v.16 no.2
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• pp.182-190
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• 2011

This paper proposes a LVRT (Low Voltage Ride Through) control strategy which should be satisfied by grid-connected wind power system when grid faults occur. The LVRT regulation indicates rules or actions which have to be executed according to the voltage dip ratio and the fault duration. Especially the wind power system has to support the grid with specified reactive current to secure the grid stability when voltage reduction ratio is over 10%. The LVRT regulation in this paper is based on the German Grid Code and full-scale variable speed wind power conversion system is considered for LVRT control strategy. The proposed LVRT control strategy satisfies not only LVRT regulation but also makes power balance between wind turbine and power system through additional DC link voltage regulation algorithms. Because it is impossible to control grid side power when the 3-phase to ground fault occurs, the DC link voltage is controlled by a generator side inverter using the DC link voltage control strategy. Through the simulation and experiment result, the proposed LVRT control strategy is evaluated and its effectiveness is verified.

• The Transactions of the Korean Institute of Power Electronics
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• v.24 no.3
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• pp.169-180
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• 2019

Grid codes for grid-connected inverters are essential considerations for bulk grid systems. In particular, a low-voltage ride-through (LVRT) function, which can contribute to the grid system's stabilization with the occurrence of voltage sag, is required by such inverters. However, when the grid voltage is under zero-voltage condition due to a grid accident, a zero-voltage ride-through (ZVRT) function is required. Grid-connected inverters typically have phase-locked loop (PLL) control to synchronize the phase of the grid voltage with that of the inverter output. In this study, the LVRT regulations of Germany, the United States, and Japan are analyzed. Then, three major PLL methods of grid-connected single-phase inverters, namely, notch filter-PLL, dq-PLL using an active power filter, and second-order generalized integrator-PLL, are reviewed. The proposed PLL method, which controls inverter output under ZVRT condition, is suggested. The proposed PLL operates better than the three major PLL methods under ZVRT condition in the simulation and experimental tests.

• Journal of Power Electronics
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• v.15 no.3
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• pp.806-818
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• 2015

Based on the inherent relationship between dc-bus voltage and grid feeding active power, two dc-bus voltage regulators with different references are adopted for a grid-connected PV inverter operating in both normal grid voltage mode and low grid voltage mode. In the proposed scheme, an additional dc-bus voltage regulator paralleled with maximum power point tracking controller is used to guarantee the reliability of the low voltage ride-through (LVRT) of the inverter. Unlike conventional LVRT strategies, the proposed strategy does not require detecting grid voltage sag fault in terms of realizing LVRT. Moreover, the developed method does not have switching operations. The proposed technique can also enhance the stability of a power system in case of varying environmental conditions during a low grid voltage period. The operation principle of the presented LVRT control strategy is presented in detail, together with the design guidelines for the key parameters. Finally, a 3 kW prototype is built to validate the feasibility of the proposed LVRT strategy.

• Journal of Power Electronics
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• v.15 no.4
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• pp.1119-1130
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• 2015

This study investigates the multi-objective fuzzy optimization of crowbar resistance for the doubly fed induction generator (DFIG) low-voltage ride-through (LVRT). By integrating the crowbar resistance of the crowbar circuit as a decision variable, a multi-objective model for crowbar resistance value optimization has been established to minimize rotor overcurrent and to simultaneously reduce the DFIG reactive power absorbed from the grid during the process of LVRT. A multi-objective genetic algorithm (MOGA) is applied to solve this optimization problem. In the proposed GA, the value of the crowbar resistance is represented by floating-point numbers in the GA population. The MOGA emphasizes the non-dominated solutions and simultaneously maintains diversity in the non-dominated solutions. A fuzzy-set-theory-based is employed to obtain the best solution. The proposed approach has been evaluated on a 3 MW DFIG LVRT. Simulation results show the effectiveness of the proposed approach for solving the crowbar resistance multi-objective optimization problem in the DFIG LVRT.

• Journal of Power Electronics
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• v.15 no.1
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• pp.235-245
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• 2015

This paper proposes a reactive current assignment and control strategy for a doubly-fed induction generator (DFIG) based wind-turbine generation system under generic grid voltage sag or swell conditions. The system's active and reactive power constrains during grid faults are investigated with both the grid- and rotor-side convertors (GSC and RSC) maximum ampere limits considered. To meet the latest grid codes, especially the low- and high-voltage ride-through (LVRT and HVRT) requirements, an adaptive reactive current control scheme is investigated. In addition, a torque-oscillation suppression technique is designed to reduce the mechanism stress on turbine systems caused by intensive voltage variations. Simulation and experiment studies demonstrate the feasibility and effectiveness of the proposed control scheme to enhance the fault ride-through (FRT) capability of DFIG-based wind turbines during violent changes in grid voltage.

• 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 KIPE Conference
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• 2019.07a
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• pp.38-40
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• 2019

태양광발전설비 보급확대에 따른 전력계통의 안정도 확보를 위해 태양광인버터 계통안정도 향상을 위한 기능들의 추가가 요구되고 있다. 계통안정도 향상 기능 중 가장 대표적인 것이 LVRT(Low Voltage Ride-Through)이고, 태양광인버터의 계통 연계에서 효과적으로 LVRT 기능을 수행하기 위한 제어기 설계 방법을 제안하고 이를 PSIM 시뮬레이션 및 실험을 통해 검증하였다.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.64 no.3
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• pp.399-404
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• 2015

This paper presents the DC voltage control method in DC link of High Voltage Direct Current(HVDC) for an offshore wind farm in Low Voltage Ride Through(LVRT) situation. Wind generators in an offshore wind farm are connected to onshore network via HVDC transmission. Due to LVRT control of grid side inverter in HVDC, power imbalancing in DC link is generated and this consequentially causes rising of DC voltage. A de-loading scheme is one of the method to protect the wind power system DC link capacitors from over voltage. But the flaw of this method is slow control response time and that it needs long recovery time to pre-fault condition after fault clear. Thus, this paper proposes improved de-loading method and we analyze control performance for DC voltage in LVRT control of HVDC for an offshore wind farm.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.13 no.4
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• pp.1814-1821
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• 2012

As a new Korean grid code which includes LVRT requirement to wind farm of which capacity is greater than 20MW is activated in 2012, this paper developed the analytical model of the Korean LVRT for the simulation based feasibility study of the wind farm interconnection into power systems. The developed model of the LVRT is verified by applying it into the performance evaluation of the wind farm interconnected power systems and the effect of Korean LVRT is analyzed through case studies considering typical disturbances of power systems.