Optimization of Damping Controller for Mitigating DC Dynamics and Internal Harmonics Causing Subsynchronous Oscillation in MMC-HVDC Connected Wind Farms

This article explores a novel subsynchronous oscillation (SSO) in the modular multilevel converter (MMC)-based high-voltage direct current (HVDC) connected wind farms. To elucidate the mechanism behind SSO, modal and participation factor analyses are performed, revealing that the dc and fundamental frequency components of the submodule voltage, as well as the MMC's dc current, are the primary contributors. Then, the impact of control strategies, circuit parameters, and operating conditions on SSO is examined using root locus analysis. Since the previous damping control method fails under the studied condition, an improved damping control strategy is proposed, incorporating optimal control signal selection to suppress SSO effectively. A systematic optimization procedure for damping controller parameters is also presented to maximize performance. Theoretical insights and the effectiveness of the proposed strategy are validated through controller hardware-in-loop (CHIL) testing.