Analysis And Suppression Of Subsynchronous Oscillation In Wind Farms Connected To MMC-HVDC

Power system stability has been the focus of the power industry for a long time.Subsynchronous oscillation,as a branch of power system stability,also affects the stable operation of the power system.The analysis of subsynchronous oscillations caused by wind farms connected to the grid via MMC-HVDC and their suppression measures are of great significance to power system safety.This thesis analyzed the subsynchronous oscillation phenomenon caused by the wind farms connected to the grid via MMC-HVDC,and studied the subsynchronous oscillation suppression measures.Based on the wind farms MMC-HVDC grid-connected system,this thesis has carried out research and innovation from three aspects:the establishment of grid-connected system simulation model,the analysis of subsynchronous oscillation mechanism,and the suppression measures of subsynchronous oscillation.Firstly,the structure of wind farms and MMC-HVDC is introduced,the control strategy of wind farms grid-side power converter is analyzed,and the model of wind farms grid-connected system via MMC-HVDC is established;then,through the generator of wind farms system,The equivalent of the grid-side power converter and MMC-HVDC system connected to the grid-side modular multi-level converter(MMC2),a simplified simulation model of the grid-connected system was established and verified by simulation.Secondly,the subsynchronous oscillation analysis method of the wind power grid-connected system is studied,the time domain analysis method and the frequency domain analysis method are compared and analyzed,the impedance model of the grid-connected system is established,and the subsynchronous oscillation analysis of the wind power grid-connected system is analyzed by the impedance analysis method mechanism.Finally,the suppression strategy of subsynchronous oscillation is analyzed and optimized.A direct current control strategy featuring fast current feedback is used,but the circulating current suppression control in the direct current control strategy is only for the double frequency component in the circulating current,ignoring the effect of the DC component in the circulating current.On this basis,this thesis is based on the impedance analysis method proposes a suppression strategy by controlling the DC component in the circulating current,gives the design method and structure block diagram of the control strategy,and simulates and verifies it.In this thesis,the impedance analysis method is used to analyze the subsynchronous oscillation mechanism of the grid-connected system,and it is deduced that the DC component in the circulating current also affects the stability of the grid-connected system.In addition,the simulation results show that on the basis of circulating current suppression control,the method of increasing the DC component in the circulating current can improve the stability of the wind power grid-connected system.