Harmonic Stability Analysis and Damping Strategy for DC system of MMC-HVDC Across Wide Frequency Range

The harmonic resonance and instability problems become prominent in recent years with the development and application of modular multilevel converter (MMC) based HVDC system. The impedance model of MMC can be used to facilitate the analysis of harmonic stability and the design of mitigation method. In this paper, the accurate dc impedance model of MMC is firstly derived based on harmonic state space (HSS) method. The internal dynamics and control actions (control loop delay included) of MMC are considered in the derivation process. With consideration of impedance characteristics of dc transmission lines, the harmonic stability for the dc system of MMC-HVDC are theoretically analyzed across a wide frequency range and the possible harmonic resonance problems are revealed. On this basis, a modified dc damping control is proposed to mitigate the harmonic resonances and improve the harmonic stability at all possible frequency points. At last, the proposed model and control method are validated by simulation results.