Title :
A switching frequency reduction and a mitigation of voltage fluctuation of modular multilevel converter for HVDC
Author :
Hak-Jun Lee ; Jae-Jung Jung ; Seung-Ki Sul
Author_Institution :
Corp. Technol. R&D Center, LSIS Co., Ltd., Anyang, South Korea
Abstract :
A switching loss of Modular Multilevel Converter(MMC) might increase drastically in HVDC system because the number of sub-module(SM) is proportional to the DC-link voltage. And, a special strategy for reducing switching frequency has been significant research issue in terms of overall operating efficiency of MMC for HVDC system. The voltage fluctuation of capacitor in SM, however, increases as the switching frequency decreases, and the capacitor with large capacitance which is the main portion of equipment cost for SM is required to mitigate the voltage fluctuation. In this paper, the switching frequency reduction strategy is proposed using the sorting method with a virtual capacitor voltage of individual SMs. In addition, this paper presents the 2nd order harmonic circulating current injection to suppress the voltage fluctuation. By numerical loss analysis, it is identified that the 2nd order harmonic current injection does not incur severe additional loss. Thanks to the harmonic current injection, the capacitance of SM capacitor could be reduced by 33% at the cost of only 0.05 % efficiency degradation in the given simulation condition. To evaluate the effectiveness of the proposed strategies, the computer simulation with 400 kV, 400MA, 221-level MMC has been performed and the results are discussed. Additionally, validity of the proposed strategies has been verified by 7-level down scaled prototype experimental setup.
Keywords :
HVDC power convertors; HVDC power transmission; switching convertors; 221-level MMC; 7-level down-scaled prototype experimental setup; DC-link voltage; HVDC system; MMC operating efficiency; MMC switching loss; SM capacitor; capacitor voltage fluctuation; computer simulation; current 400 MA; modular multilevel converter; numerical loss analysis; second-order harmonic current injection; sorting method; submodule number; switching frequency reduction strategy; virtual capacitor voltage; voltage 400 kV; voltage fluctuation mitigation; Capacitors; Fluctuations; Harmonic analysis; Sorting; Switches; Switching frequency; Voltage fluctuations; HVDC; MMC; Modular Multilevel Converter; Switching frequency reduction;
Conference_Titel :
Energy Conversion Congress and Exposition (ECCE), 2014 IEEE
Conference_Location :
Pittsburgh, PA
DOI :
10.1109/ECCE.2014.6953433