Analysis and mitigation of AC coupling effects on overhead line of modular multilevel converter (MMC) based HVDC transmission system

Nowadays, the Modular Multilevel Converter (MMC) based HVDC has been considered as a promising technology to meet the increasing electric power demand. However, constructing new transmission lines becomes more and more difficult because of environmental issues. Consequently, the newly constructed DC lines have to share the right of way with existing AC transmission in close proximity. Meanwhile, as the number of these parallel DC/AC lines increases, many problems related to AC coupling effects on DC line have been reported in several studies. The AC voltage and current are induced on DC line and they have a harmful influence of HVDC system, such as saturation of converter transformer and noise emissions. However, the previous studies related to this issue only deals with the analysis of the effects on MMC with old-fashioned control method or the mitigation methods requiring additional hardware. In this paper, considering the recently developed decoupled control strategy, a new analysis of the coupling effects on MMC is investigated and a novel control method to mitigate the effects is proposed. By applying the proposed control scheme, MMC can operate stably without any interruption from the AC coupling effects. Finally, the full scale computer simulation results and the down scaled experimental results demonstrate the validity of the proposed control schemes.