Modular multilevel converter(MMC)is suitable for grid-connected interface for medium and high voltage power grids. For the potential sub-synchronous and super-synchronous oscillation problem in photovoltaic(PV)medium voltage direct current(DC)power collection system

the positive and negative sequence impedance models of MMC converter are systematically established and the basis of the stability analysis for grid-connected MMC inverters is provided from the frequency domain. Positive and negative sequence impedance overlaps with the grid impedance

which predicts the sub-synchronous and super-synchronous oscillation of the PV medium-voltage DC power collection system under series compensation and verifies the consistency of frequency domain impedance and time domain waveform analysis. By controlling the parameter optimization

the sub-synchronous and super-synchronous oscillation phenomenon of the MMC under series compensation is effectively suppressed.