针对光伏虚拟同步发电机（virtual synchronous generators, VSG）经高压直流输电（Modular Multilevel Converter based high voltage direct current, MMC-HVDC）送出系统所存在的次同步振荡（subsynchronous oscillation, SSO）风险，建立系统线性化数学模型。本文提出一种改进型虚拟同步控制策略，利用特征值法分析当功率发生扰动时，光伏虚拟同步发电系统有功控制环节、无功控制环节、虚拟电感、MMC-HVDC桥臂电感、电流矢量控制环节对系统SSO的阻尼特性和频率特性均会产生影响，并在PSCAD/EMTDC仿真平台进行验证。结果表明，引入VSG控制后，在外送系统中发生功率扰动的过程中，存在光伏VSG和MMC-HVDC共同参与的次同步振荡模式；在光伏VSG侧，有功调频系数Kf过大或虚拟电感Lv过小均会导致系统失稳；MMC-HVDC侧，电压环积分系数Ki4增大会导致系统失稳，而增大桥臂电感Lg会增强系统稳定性。

Aiming at the problem of subsynchronous oscillation(SSO) when Photovoltaic virtual synchronous generators (VSG_PV) are sent out of the system through modular multi-level converter based on high voltage direct current (MMC-HVDC), a linearized mathematical model of the system is established. This paper proposes an improved virtual synchronous control strategy. The eigenvalue method is used to analyze that when the power is disturbed, the active control link, reactive control link, virtual inductance, MMC-HVDC bridge arm inductance, and current vector control link of the photovoltaic virtual synchronous power generation system will all affect the damping characteristics and frequency characteristics of the system SSO, and it is verified on the PSCAD/EMTDC simulation platform. The results show that, with the introduction of VSG function, there is a sub-synchronous oscillation mode in which photovoltaic VSG and MMC-HVDC participate in the process of power disturbance in the outgoing system. On the photovoltaic VSG side, the active frequency modulation coefficient Kf is too large or the virtual inductance Lv is too small, which will lead to system instability; On the MMC-HVDC side, the increase of voltage loop integral coefficient Ki4 will lead to system instability, while the increase of bridge arm inductance Lg will enhance system stability.