Research On Characteristic And Suppressing Methods Of Subsynchronous Oscillation For System With DFIG-based Wind Farms

With the rapid growth of grid-connected capacity of wind power and other new energy sources~as well as the development and widespread application of the series capacitors,ultra-high voltage power transmission and high-power electronics technology,the new energy power system gradually presents the characteristics of multi-source,multiple convertors,complex structure and multi power electronics,which have led to the new problem of sub-synchronous oscillation and are becoming more and more complicated and highlighted.This paper focuses on two types of new sub-synchronous oscillation problems that occur in power system with the wind farm,namely,the sub-synchronous control interaction(SSCI)in the doubly fed induction generator(DFIG)-based wind farm with fixed series compensation and the SSO problem of wind-thermal bundling system with high voltage direct current(HVDC)transmission.The following studies are carried out:Firstly,the mathematical model of DFIG is established.The interacting path of SSCI which is the interaction between DFIG-based wind turbine generators and series compensated transmission is analyzed.Then the mechanism and characteristics of two types of SSO problems concerned with wind-thermal bundling system with HVDC transmission.One is the sub-synchronous torsional interaction between thermal power and HVDC.The other is sub-synchronous interaction between wind power and thermal power.Thus the studies abovelay the foundation for the subsequent research on sub-synchronous oscillation suppression.Aimed at the SSCI in the DFIG-based wind farm with fixed series compensation,the mathematical criteria of SSCI occurrence conditions are deduced according to the action path of SSCI,and the related characteristics and effectiveness are verified by simulation.Based on this condition,a SSCI event-triggered supplementary damping control method is proposed.For single resonant frequency,a method to extract sub-synchronous frequency suitable for damping control is proposed.At the same time,the relevant parameters are reasonably designed and the effectiveness of control strategy and the adaption of control parameters is verified by simulation.Aiming at the two sub-synchronous oscillation problems of the wind-thermal bundling system with HVDC transmission,corresponding suppression method is designed.The damping sensitivity of the acting path is analyzed according to the mechanism of sub-synchronous torsional interaction of thermal power caused by HVDC.The results indicates that the electrical damping is the most sensitive to the controlled variable of the firing angle.Based on it,an SSO mitigation method by interdicting sub-synchronous components in HVDC control link.The relevant parameters are reasonably designed and the feasibility and effectiveness of the method are verified through damping scanning and time-domain simulation.Aiming at the problem of sub-synchronous oscillation divergence of thermal power units caused by the interaction between wind power and thermal power,sub-synchronous current counter-injection method based on shunted voltage sourced converter(VSC)is proposed to suppress the wind-thermal interaction,the sensitivity of the control parameters is analyzed,and the effectiveness of the suppression method are verified through time-domain simulation.