Small Signal Stability Analysis Of Grid-Connected D-PMSG Via VSC-HVDC System

In the development of wind power technology,due to restrictions on land resources and wind energy resources,large-capacity and long-distance offshore wind power will be a trend in the future development of wind power.With the unique advantages of direct-drive permanent magnet synchronous generators(D-PMSG)and voltage source converter based high voltage direct current(VSC-HVDC),direct-drive wind farms connected to grid by VSC-HVDC have become one of the future development trends of offshore wind farms.The sub-synchronous oscillations(SSO)will be caused by interaction between D-PMSG and VSC-HVDC in the system which is rarely studied in the references for this system structure.Direct-drive wind farms connected to grid by VSC-HVDC is taken as the research object,and the SSO problem under the two system structures of single machine equivalent and multi-machine equivalent is analyzed,focusing on the SSO characteristics and the influence of multi-machine equivalent on oscillation characteristics in this paper.The specific research contents are as follows:Mathematical model of D-PMSG and VSC-HVDC system is constructed in this paper,in which the permanent magnet generator model,the shafting model,the back-to-back converter model and its corresponding control model are included in the model of D-PMSG system.The simplified single-ended model is adopted in the VSC-HVDC system,and the constant AC voltage control strategy is used in rectifier station.The small-signal model is established by taken the D-PMSG connected to grid by VSC-HVDC as the research object.Combined with the PSCAD/EMTDC simulation platform,the eigenvalue analysis method is used to study the SSO characteristics of the system.Among them,the equivalent aggregation model is adopted in the D-PMSG and is represented by a single equivalent direct-drive wind turbine to analyze the influence of system structural parameters and controller parameters on the frequency and damping characteristics of SSO.The results show that the structural parameters and control parameters related to the DC side of the D-PMSG have a great influence on the stability of the system and the changes of other system parameters will affect the system damping.The eigenvalue analysis of the grid-connected system in which two equivalent D-PMSG are connected to the VSC-HVDC is compared with the single-machine aggregation model,focusing on the effects of the aggregation characteristics of wind farms and the position of wind turbines on the system stability.The results show that the system oscillation modes will be changed by the aggregation of the wind farms,and the system damping will be reduced by the increase of the grid-connected distance of the wind farms.