Research On Subsynchronous Oscillation Characteristics Of Direct-drive Wind Farms And MMC-HVDC

With the rapid development of DC transmission technology,MMC-HVDC has gradually become one of the mains method of wind farms with grid.But the Subsynchronous Oscillation(SSO)issue caused by the interaction of Direct-Drive Permanent Magnet Synchronous Generator(D-PMSG)via MMC-HVDC turn out to be more prominent.So,there is necessary to study the SSO characteristics of wind farms connected to MMC-HVDC.In the paper,the eigenvalue analysis method and time-domain simulation analysis method are used to analyze and study the SSO mechanism and characteristics of the D-PMSG and MMC-HVDC.The research results are as follow:(1)Building the small signal dynamic model of model 1 and model 2.Model 1 is equivalent to the wind farm as an AC source,which is used to study the influence of a single MMC converter on SSO,while the model 2 is used to study the impact of D-PMSG converter and MMC converter on SSO,so the model 2 completely shows the structure of the wind turbine.Secondly,the dynamic model of MMC,D-PMSG and AC lines is built using small signal theory.For the model 2,the small-signal dynamic models of D-PMSG and MMC-HVDC are based on different node voltages,the general expression of the dynamic interface between them is derived,and the dynamic matrix and dynamic equation of the D-PMSG and MMC-HVDC are obtained.Finally,the small signal dynamic model of whole system is constructed.(2)Analyzing the SSO characteristics of model 1 and model 2.The eigenvalues of the two models are calculated,and the corresponding oscillation mode and oscillation frequency are obtained.Then,calculating the participation factors of each oscillation mode and the strong correlation variables affecting the SSO are obtained.At the same time,the accuracy of the dynamic model is verified in the electromagnetic transient simulation software PSCAD/EMTDC.Next,calculating the eigenvalue sensitivity analysis of system structural parameters and controller parameters to find out the controller parameters that dominate each osscillation mode.Then,figure out the eigenvalue sensitivity analysis of MMC-HVDC and D-PMSG controller parameters to find the controller parameters that dominate each oscillation mode.The results show that there is a coupling relationship between SSO modes in model 1: the same controller parameter will affect multiple SSO modes,and the same SSO mode will also be affected by multiple control parameters.The coupling effect makes the damping of SSO mode change in the same or different direction when the control parameters change.In model 2,SSO mode is also affected by multiple control parameters,and SSO is more affected by MMC controller than DPMSG.(3)Analyzing the influence of circulation suppression controller of MMC-HVDC on SSO.Firstly,the structure of MMC-HVDC controller and the dynamic model of the circulation suppression controller are introduced,and the circulation suppression controller is added to model 1.On this basis,calculating out the eigenvalue calculation and the participation factor analysis to find out the strong correlation variables that affect SSO.Then the eigenvalue sensitivity of each controller parameter is calculated to get the dominant controller parameters of SSO,and the influence of these dominant controller parameters on SSO is analyzed.The results show that the proportion and integral parameters of the circulation suppression controller have different effects on SSO.Finally,the model is established in PSCAD/EMTDC to prove that the circulation suppression controller of MMC-HVDC can suppress SSO.