Impedance-based Study On Sub-synchronous Oscillation Caused Of VSC-HVDC With Doubly-fed Induction Generator Grid-connected System

Most of the wind farms are built in remote areas,even at sea.Therefore,the power output of the wind farm must be transmitted in long distance through the VSC-HVDC transmission technology.The DC transmission equipment will cause the sub-synchronous oscillation(SSO)in the operation process,also called the sub-synchronous torsional interaction(SSTI)caused by the device.It affects the safe operation of the power system.Therefore,the study of sub-synchronous oscillation of doubly-fed wind generator through VSC-HVDC grid-connected system is carried out.In this paper,firstly,The mathematical model of doubly-fed wind farm through VSC-HVDC grid-connected system is derived.The detailed time domain simulation model of doubly-fed wind farm connected to grid through VSC-HVDC is established based on PSCAD/EMTDC to verify the above mathematics.Then,the impedance modeling method in two-phase dq coordinate system is used to treat the part of plays a decisive role in the SSTI caused by the DFIG with VSC-HVDC grid-connected system as two interconnected subsystems.The small signal impedance models of DFIG grid-side converter and AC side of rectifier station of VSC-HVDC system are derived,and the correctness of the impedance model is verified.Finally,The Nyquist curves under different factors are drawn to determine the stability of interconnected systems and the influence of various factors,including external circuit parameters and internal control parameters,on sub-synchronous oscillation is analyzed.The conclusion is verified by the detailed time domain simulation model of the DFIG with VSC-HVDC grid-connected system.And then,according to the conclusion obtained,the corresponding SSTI suppression method was found.