A Supplementary Damping Control for MMC-HVDC System to Mitigate the Low-Frequency Oscillation Under Low Inertia Condition

With the increasing penetration of renewable energy, the inertia of AC system gradually decreases. When modular multilevel converter (MMC) based high voltage DC transmission (HVDC) system is connected to a low inertia AC system, the low-frequency oscillation may occur due to the interaction between MMC and synchronous generator (SG). To investigate this issue, a detailed small-signal model considering the dynamics of MMC and SG is developed and the corresponding eigenvalue analysis is conducted. Based on the developed model, the impact of the inertia on the low-frequency oscillation of MMC-HVDC system is studied, and the dominant low-frequency mode is investigated by participation factor and control parameter sensitivity analysis. From the results, it is found out that there exists close control interaction between SG and MMC under low inertia condition. To damp the low-frequency oscillation of MMC-HVDC system under low inertia condition, a supplementary damping control (SDC) approach is proposed, the parameters of the SDC approach are tuned by pole-assignment approach. Finally, the damping effect of the SDC approach is validated by theoretical analysis and simulation study in PSCAD.