Research On The Sub-synchronous Oscillation Analysis And Mitigation Strategies Of The Direct-drive Based Wind Farm Connected To The Weak AC Power Grid

With the rapid development of wind power,the installed capacity of wind power is on the rise.Due to the limitation of land resources and wind energy resources,wind farms are often located in remote areas,which makes the connected system gradually show the characteristics of weak AC power grid.In recent years,the interaction between direct-drive based wind farm and weak AC power grid frequently occurs in Western China,which causes sub synchronous oscillation(SSO),which poses a great threat to the stable operation of the system.In this paper,the SSO problem caused by the direct-drive based wind farm connected to a weak AC power grid is studied as follows.The mathematical model of the direct-drive based wind farm connected to a weak AC power grid is established,and short circuit ratio(SCR)is used to describe the strength of AC power grid.Based on the above model,the interaction mechanism between the grid side converter of direct-drive permanent magnetic synchronous generator(D-PMSG)and a weak AC grid is revealed from the physical path.In the mechanism,the subsynchronous current generated by the direct-drive based wind farm connected to a weak AC power grid is first reflected on the output voltage of the grid side converter of the D-PMSG through the PI controller of the inner current loop of the grid side converter of the D-PMSG,so that the grid side converter of the D-PMSG outputs the subsynchronous voltage and generates a new subsynchronous current.The new subsynchronous current forms a positive feedback with the original subsynchronous current,which enhances the oscillation and eventually leads to the instability of the whole system.To mitigate the SSO problem of direct-drive based wind farm,this paper proposes two mitigation strategies from two perspectives.On the other hand,from the power side,the SSO mitigation strategy of direct-drive based wind farm based on active disturbance rejection control(ADRC)theory is proposed.In this strategy,the ADRC controller is designed to replace the current inner loop PI controller of grid-side converter of D-PMSG,and the mitigation mechanism of the ADRC controller is derived mathematically.The simulation results show that the proposed control strategy not only has good mitigation effect on SSO,but also has strong robustness and adaptability.On the other hand,from the grid side,from the perspective of the grid side,an additional control mitigation strategy based on static synchronous compensator(STATCOM)for SSO of direct-drive based wind farm is proposed.In the additional control strategy,STATCOM first detects the sub-synchronous current signal,and then outputs the sub-synchronous current with the phase opposite to the signal according to the extracted sub-synchronous current signal,thereby canceling the sub-synchronous current in the direct-drive based wind farm and avoiding the divergence of the sub-synchronous current,and ultimately achieve the purpose of mitigating SSO.The effectiveness of the proposed method is verified by time-domain simulation.