| Due to the ability of stabilizing voltage and frequency,modular multi-level converter based HVDC(MMC-HVDC)has become an important way of delivering new energy.A number of domestic and international MMC-HVDC projects has been already in operation or are being planned.In recent years,broadband oscillations were reported in certain number of wind power delivering via MMC-HVDC projects during commissioning or operational phase,which attracted a great deal of attention from industry and academia.This thesis is a systematic study of the mechanism and suppression strategy of broadband oscillations in wind farms delivering via MMC-HVDC systems: detailed impedance models for permanent magnet synchronous generator(PMSG),(double fed induction generator)DFIG and MMC-HVDC were established,impedance broadband were divided,the negative damping impedance characteristics in each band were analyzed,the dominant factors of oscillation in each band were revealed,and an oscillation suppression strategy based on impedance reshaping was proposed.The main work and innovative results are as follows:Detailed impedance models of PMSG considering machine-side converter(MSC)model and DFIG considering power outer loop were established respectively,the frequency distribution pattern of the small signal components at various locations within the wind turbine was clarified.The DC bus dynamic link was found to be the MSCgrid-side converter(GSC)impedance coupling path of the PMSG and DFIG,and the power outer loop provides a new impedance coupling path for DFIG.These coupling paths above have a significant impact on the impedance characteristics of the wind turbine unit and on the stability analysis.The frequency domain mathematical models of PMSG,DFIG,MMC-HVDC were established respectively,for the dependence between their broadband impedance characteristics and their multi loop control system.The established models considering different types of power electronic devices and their controllers in different bandwidth.The dominant factors of the impedance characteristics in various band were analyzed,the impedance band division method of PMSG,DFIG and MMC-HVDC was proposed,and the mechanism of the negative damping characteristics due to overlapping effects between controllers in each frequency band was found.For the problems in the delivering system that oscillation coupling frequencies are many,the dominant oscillation factors are difficult to be located,and oscillation mechanism is unclear,a method to determine the dominant factors of oscillation in different frequency bands of PMSG,DFIG and MMC-HVDC was proposed and the mechanism of broadband oscillation of the wind farm delivering via MMC-HVDC system was revealed.Based on the maximum peak Nyquist criterion,the problem that it is difficult to accurately judge the system stability under the power fluctuation of wind farm was effectively solved.For the negative damping impedance characteristics of PMSG,DFIG and MMC-HVDC that cause broadband oscillation of the system,an impedance reshaping strategy based on control structure and parameter optimization and a unified iterative optimization method considering multiple controller parameters with overlapping frequency bands were proposed.This method can consider the interaction of impedance characteristics between different frequency bands,and take into account the requirements of system oscillation suppression and fault ride through,so as to ensure the stable operation of the system under complex working conditions.The research results of impedance modeling,oscillation mechanism and suppression strategy have been effectively verified by the full electromagnetic transient simulation platform of new energy delivering via MMC-HVDC system.On this basis,aiming at the oscillation problems of Zhongdu station and Kangbanor station in Zhangbei VSC-HVDC power grid project,the oscillation mechanism was analyzed,the oscillation leading equipment was determined,the oscillation suppression strategy was proposed.And the on-line reproduction and on-line suppression field tests of oscillation were successfully carried out,which effectively solves the oscillation problems of practical projects and further verifies the correctness and effectiveness of the research results of this thesis. |
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