Study on the Effect of Cancelling the DC Smoothing Reactor on the Stability Analysis and Propagation Characteristics of SSO in MMC-HVDC System

Modular multilevel converters(MMC) based high-voltage DC(HVDC) transmission is widely used for offshore wind farm integration. In some projects, the DC smoothing reactor on the DC side of the HVDC system are cancelled due to cost and space limitations, and the effects of that on the small-signal stability issues of the MMC-HVDC system in the sub-synchronous range will be studied in this paper. Firstly, the effect of DC smoothing reactor on the single input and single output(SISO) admittance of MMC is discussed, then the sub-synchronous oscillation propagation characteristics of the MMC are studied, and the conclusions are interlinked, furthermore, an oscillation propagation mitigation control strategy based on the propagation characteristics is proposed in this paper, which can replace the DC smoothing reactor to some extent. Finally, a case is studied in a typical offshore wind farm integration system in Matlab/Simulink.