Effects of Control on the DC Impedance of Modular Multilevel Converters

The dc impedance of modular multilevel converters (MMCs) and two-level voltage-source converters (TL-VSCs) in open-loop control mode, ac current control mode and dual-loop control mode are derived and compared separately based on multiharmonic linearization method. Then the effect of control loops on the dc impedance of MMC are further analyzed. Moreover, this paper provides a deep discussion about the effect of four circulating current suppressing controller (CCSC) strategies in terms of impedance and stability. It is revealed that dc impedance of MMC differs from TL-VSC in most control modes due to the complex internal dynamics. It also turns out that dc voltage control loop introduces negative damping effect which can be reduced by increasing proportional and decoupling coefficients of CCSC or decreasing proportional coefficient of dc voltage control loop. The time-domain simulations are provided to verify the theoretical analysis.