Mechanism Analysis of MMC Oscillation Based on Damping Path Decomposition

In modern power systems with a high penetration of power-electronic-interfaced resources, grid-following (GFL) Modular Multilevel Converters (MMCs) face a risk of sub-synchronous oscillations caused by the coupling of multiple control loops. Such oscillations are driven by converter control, with damping jointly shaped by dynamic coupling among the phase-locked loop (PLL), DC voltage control (DVC), reactive power control (RPC), and other inner loops. Conventional impedance-based analysis cannot trace such oscillations back to specific control loops, nor can it characterize their interaction. To address this gap, this paper establishes a small-signal model of the GFL MMC and characterizes its second-order DC voltage control (DVC) loop by analogy with the rotor motion equation of a synchronous machine, thereby enabling an analysis of how the other control loops affect the damping of the DVC. Based on this model, a damping path decomposition method is proposed to quantitatively decompose the system electrical damping into four contributions: the intrinsic effect of the DVC, the influence of the PLL on the DVC, the influence of the RPC on the DVC, and the effect arising from the interaction between the PLL and RPC. Analysis results indicate that the DVC itself possesses a certain degree of positive damping, RPC provides positive damping, while the PLL and its interaction with RPC contribute negative damping to the DVC loop. The influence of control parameters and operating conditions on the overall system damping is then investigated. The predicted trends are confirmed through EMT simulation, providing a theoretical basis for parameter tuning and stable MMC operation under weak-grid conditions.