Mechanism and Suppression of Mid-High Frequency Oscillation for Wind Farms Integrated With GFM-MMC System

Grid-forming control, due to its active support ability, emerges as a feasible method for renewable energy transmission. However, the oscillation characteristics of wind farms integrated with grid-forming modular multilevel converter (GFM-MMC) system have rarely been researched. This paper reveals the mechanism of mid-high frequency oscillations in GFM-MMC system, and proposes a disturbance estimation-state prediction (DE-SP) based control method which achieves delay compensation and disturbance suppression. Firstly, the impedance model of wind farms and GFM-MMC considering frequency coupling effect is established, the impedance characteristics and oscillation mechanism are obtained. Subsequently, a DE-SP based control method is proposed, where a state estimator reconstructs system states, a filter estimates system disturbances, and a state predictor obtain future states. Then, the control structure of GFM-MMC is designed, and the impacts of control parameter on impedance characteristics is analyzed. Finally, the RTDS and Compact RIO-based hardware-in-loop (HIL) experimental is conducted. Compared with feedforward damping and Smith-based method, the proposed DE-SP method can not only suppress the mid and high frequency oscillations, but also exhibits strong robustness against disturbances and estimated delay compensation, which can significantly improve the stability of GFM-MMC system.