Inside-wind-farm/Wind-farm-grid Sub-synchronous Oscillation Characteristics Analysis Of The DFIG-based Wind Farm Integrated Into Grid Through VSC-HVDC System

Wind power generation,as one of the most widely used and fastest developing new energy generation technologies,has been developed and applied on a large scale all over the world.Studies have shown that the sub-synchronous oscillation(SSO)problem becomes more and more serious when the wind farm is connected to the grid by the voltage source converter based on high voltage direct current(VSC-HVDC)transmission system.As an important scenario of grid-connected wind farms via VSC-HVDC systems,the SSO oscillation problem of the double-fed wind farm through the VSC-HVDC system can not be ignored.At present,studies on SSO in this scenario mainly focus on the interaction between DFIG-based wind farm and VSC-HVDC,while few studies focus on the interaction between DFIGs in wind farm.In light of this,taking the doubly-fed wind farm equivalent to three doubly-fed wind turbines,inside-wind-farm/wind-farm-grid oscillation characteristics in the DFIG-based wind farm integrated into grid through VSC-HVDC system are studied as follows:Firstly,the whole system small-signal model of three equivalent doubly-fed wind turbines integrated into grid through VSC-HVDC system is estblished,in which each doubly-fed wind turbine represents the equivalent model of a single doubly-fed wind turbine.For the convenience of analysis,the doubly-fed wind farm is divided into doubly-fed wind farm system and VSC-HVDC system at the common bus of the two systems.The dynamic mathematical model of doubly-fed wind farm system,VSC-HVDC system and the interface model between the two systems are established respectively.Based on these dynamic mathematical models,the whole system small signal model of the doubly-fed wind farm via VSC-HVDC system is established.Then,based on the established small signal model of doubly-fed wind turbines integrated into grid through VSC-HVDC system,eigenvalue analysis and participation factor analysis are used to analyze the SSO mode and the participation factors of each SSO mode of the three doubly-fed wind turbines through the VSC-HVDC system,and the time domain simulation verification is carried out on the PSCAD/EMTDC simulation platform to verify the correctness of eigenvalue and participation factor analysis results.The results show that three equivalent doubly-fed wind turbines through the VSC-HVDC system have inside-wind-farm SSO caused by the interaction between the doubly-fed turbines in the wind farm and wind-farm-grid SSO caused by the interaction between the doubly-fed turbines and the VSC-HVDC system.Finally,based on the result of the eigenvalue analysis and participation factor analysis of three doubly-fed wind turbines through the VSC-HVDC system,the damping characteristics of inside-wind-farm SSO mode and wind-farm-grid SSO mode on the variation of doubly-fed wind turbine parameters and VSC-HVDC system parameters are analyzed,and the time domain simulation verification is carried out on PSCAD/EMTDC simulation platform.The analysis results show that,the damping variation trend of the two SSO modes is the same with the variation of parameters of the doubly-fed wind turbine,and the variation of parameters of VSC-HVDC system mainly affects the wind-farm-grid SSO mode damping,but has no obvious influence on inside-wind-farm SSO mode damping.