Analysis And Mitigation Of Sub-synchronous Oscillation In Power System With DFIG-based Wind Farms

With the rapid development of new energy sources,the installed capacity of wind power continues to increase.There is a risk of sub-synchronous oscillations(SSO)when large-scale wind farms transmit power through series compensation techniques.Among the wind power generators,the doubly-fed induction generator(DFIG)is a mainstream type.The stability problem of DFIG is more serious and can not be ignored.This paper focuses on the SSO problem of the DFIG-based wind farm connected to the series-compensated transmission system,and the following researches are carried out:The model components of the DFIG-based wind farm transmitted through the series-compensated transmission system are established respectively.The mechanism of the SSO in wind power system and the time-varying characteristics of the oscillation frequency are analyzed.The applicability of different equivalent modeling methods for wind farms is studied.The equivalent precisions of the controlled source equivalent method,the equivalent parameter equivalent method and the single impedance equivalent method under different wind farm structures are compared,and the equivalent modeling guidelines suitable for SSO analysis are given.According to the mechanism of the SSO in wind power system,a rotor-side converter mitigation strategy of DFIG based on active disturbance rejection control(ADRC)is proposed.Sub-synchronous disturbances of the system are evaluated in real time and feedback compensated with extended state observer,and the SSO under different system operating conditions are suppressed.The time domain simulation results show that the designed DFIG mitigation strategy based on ADRC is more suitable for wind power generation systems with strong output fluctuations and strong disturbance factors.The mitigation effect is not affected by the time-varying characteristics of the oscillation frequency.This strategy can effectively mitigate the sub-synchronous control interaction between the DFIG controller and the fixed series compensation.Moreover,ADRC does not require complicated parameter tuning,and good mitigation effect and strong robustness can be achieved in a wide range of parameter variations.Combined with the interactive characteristics of SSO in wind power system,a wide-band grid-side centralized mitigation measure based on static var generator(SVG)is proposed.Based on the power compensation characteristics of SVG,a damping modulation strategy for extracting wide-band sub-synchronous signals is designed.And this strategy is applicable to the SSO mitigation problem of time-varying oscillation frequency.The time domain simulation results show that according to the sub-synchronous signal in the line,SVG can offset the sub-synchronous energy by generating the corresponding inverting sub-synchronous power.Thus,the interaction between the DFIG-based wind farm and the series-compensated line is blocked,and the SSO is mitigated effectively.