Research On Subsynchronous Oscillation Characteristics Of Grid Connected System In Doubly Fed Wind Farm

With the rapid development of wind power generation technology in today's era,large-scale long-distance transmission of large capacity wind power becomes inevitable,and some transmission problems also appear.Although series compensation capacitor technology can improve the transmission efficiency,it will suffer from the serious threat of Subsynchronous Oscillation(SSO)in power system.At present,many subsynchronous oscillations of power system caused by grid connected fans have occurred at home and abroad.Therefore,it is very important to analyze the influence of wind power grid connected system on subsynchronous oscillation.This paper focuses on the influence of DFIG connected to series compensation transmission line on the stability of grid connected system.Firstly,the induction generator effect(IGE)and subsynchronous control interaction(SSCI)are considered to be the root cause of the subsynchronous oscillation of doubly fed wind farms connected to the grid through series compensation lines,which is purely electrical process and has nothing to do with the shafting movement.According to this characteristic,this paper studies the sub synchronous oscillation mechanism of the grid connected system of doubly fed wind farm based on the impedance analysis method,and establishes the impedance model of each module in the system.At the same time,the large wind farm is equivalent to the two sub wind farms of the far end and the near end.Secondly,with the remaining conditions unchanged,the frequency characteristic curves of different system equivalent impedances are drawn by changing the seriescompensation degree of the transmission line,the wind speed ratio of the far and near end fans,the proportion coefficient of the current inner ring at the rotor side of the fans and the cable parameters between the wind farms.Combined with the stability criterion,the influence of the above variables on the subsynchronous oscillation of the grid connected system is judged.Then,on the PSCAD/EMTDC software simulation platform,the grid connected system model is built,and the time-domain simulation of the model is carried out to further analyze the influence of the subsynchronous oscillation of the grid connected system of the doubly fed fan,and verify the correctness of the impedance analysis method and the applicability of the model.At last,a virtual resistance is introduced into the rotor side of the DFIG,which makes the equivalent impedance of the grid connected system increase under the subsynchronous frequency,so that the vibration can be restrained.The simulation results show that the strategy is effective.