跟网/构网型直驱风场经柔直送出系统的振荡分析与抑制策略OA

For the grid-following/grid-forming(GFL/GFM)direct-drive wind farm integration system via modular multilevel converter-based high-voltage direct current(MMC-HVDC)transmission,current research has mainly focused on scenarios where GFL/GFM converters are directly connected to the power grid.There is a lack of equivalent impedance modeling methods suitable for the GFL/GFM direct-drive wind farm integration system via MMC-HVDC.Meanwhile,existing oscillation suppression strategies are primarily designed for the standalone grid-connected operation of GFM converters,making them difficult to be fully applied to the aforementioned wind farm integration system.In addition,the existing strategies are targeted at relatively single operating conditions and may have limitations in handling multi-condition scenarios caused by wind speed variations.To address these issues,this paper first considers the frequency coupling phenomenon within the transmission system and establishes an equivalent impedance model of the system.Simultaneously,an oscillation suppression strategy based on grid-connected current feedforward is proposed,and its effectiveness is verified using the Matlab/Simulink simulation platform. Firstly,based on the impedance models of GFL direct-drive wind turbines,GFM direct-drive wind turbines,AC transmission lines,and the sending-end MMC,the GFM permanent magnet synchronous generator(GFM-PMSG),MMC,and AC transmission lines are regarded as an integrated unit,collectively referred to as the parallel system.The equivalent impedances of the GFL-PMSG and the parallel system are calculated respectively.The accuracy of the established impedance models is verified by comparing the model-derived results with the simulation frequency sweep outcomes.Then,based on the impedance analysis method,a stability criterion for the transmission system is derived. Subsequently,this section conducts further stability analysis of the transmission system from two aspects:the capacity proportion of GFM wind turbines and the wind speed of the wind farm.On the one hand,when the capacity proportion of GFM-PMSG is relatively low,it may introduce new oscillation risks to the system in the frequency band near the power frequency.The oscillation frequency generated in this case is significantly different from that induced by the standalone grid connection of GFL-PMSG.Although increasing the proportion of GFM-PMSG can avoid oscillations near the power frequency,it will raise the capital investment cost of wind farm construction.On the other hand,in the wind farm integration system via MMC-HVDC,when the wind farm is fully equipped with GFL wind turbines,the system faces the risk of oscillation instability under high wind speed conditions.The integration of a certain capacity of GFM wind turbines can ensure the stable operation of the transmission system under high wind speeds.Nevertheless,when the proportion of GFL wind turbines is relatively high,the transmission system will be exposed to new oscillation risks under low wind speed conditions. Finally,by analyzing the influence of disturbance propagation paths of each component on the system impedance,this paper identifies the dominant propagation paths that exert a primary impact on the oscillation-prone frequency bands.An oscillation suppression strategy based on grid-connected current feedforward is proposed,whose core principle is as follows:a reverse disturbance signal is superimposed on the output channel of the current inner loop,and this signal is generated by processing the dq-axis components of the inductor current disturbance through the current inner loop.The control structure is improved through the above method to reshape the system impedance and enhance the system damping in the oscillation-prone frequency bands.Then,the Matlab/Simulink simulation platform is employed to verify the effectiveness of the proposed suppression strategy for oscillations arising under different scenarios.The research results indicate that the proposed grid-connected current feedforward-based oscillation suppression strategy can simultaneously suppress the oscillations induced by both GFL-PMSG and GFM-PMSG in the transmission system,and achieve oscillation suppression under multiple operating conditions of the transmission system when wind speed changes.