Study on Sub-synchronous Oscillation of Grid-connected VSC via Series Compensation Lines

This paper mainly concentrates on the mechanism studies of the sub-synchronous oscillation (SSO) caused by the grid-connected VSCs when integrated into the grid via series compensation lines (SCLs). To further analyze the impacts of VSC's terminal voltage feed-forward (TVF) on this SSO, a linearized model of VSC in current control timescale (less than 20ms) is proposed. This model divides the current control loops of VSC into two loops, namely the current controller loop and the TVF loop. By analyzing these two loops, it is able to reveal the mechanisms of the SSO caused by VSC via SCLs. The analytical results indicate that the TVF will introduce a negative damping into the system through the TVF loop when VSC integrated into the grid via SCLs. Besides, with the increase of the TVF filter bandwidth, the negative damping introduced by TVF will further deteriorate the stability of the system. Finally, the correctness of the theoretical analyses is validated by time domain simulations in MATLAB/Simulink.