High-Frequency oscillation mechanism and suppression strategy for renewable energy integration via VSC-HVDC systems

High-frequency oscillations (HFOs) have emerged as a recurrent phenomenon in large-scale renewable energy integration via VSC-HVDC transmission projects. Based on HFOs at Kangbao Station of the Zhangbei VSC-HVDC Project in China, this paper identifies how time delays in the conventional closed-loop control system of Modular Multilevel Converter (MMC) induce negative damping, thereby triggering HFOs. Then, a novel current control strategy combining predictive feedforward and deviation feedback is proposed to suppress HFOs. The strategy employs the nominal closed-loop transfer function of the conventional current inner loop as the current-tracking reference model to generate the desired valve-side current dynamic response. Meanwhile, a predictive feedforward controller generates the main modulation signal. The deviation between the desired and actual valve-side current is fed into a compensator to generate the compensation modulation signal, forming a feedback closed-loop that ensures rapid convergence of the actual response to the desired response. Furthermore, a phase-lead compensator is integrated into the output path of the current tracking model to provide supplementary damping in the high-frequency range. This proposed strategy can eliminate negative damping of the converter stations while preserving medium-to-low frequency performance, robustness, and disturbance rejection capability. Simulation results demonstrate that the proposed strategy can effectively suppress HFOs under various operating conditions and time-delay uncertainties, while ensuring the VSC-HVDC system operates stably under normal conditions and meets fault ride-through (FRT) requirements under fault conditions.