Mechanism And Characteristics Of Forced Subsynchronous Oscillations In Direct-drive Pmsg Based Wind Farms Excited By Interharmonics

With the rapid development of wind power generation,the power system is facing various problems of power quality and broadband oscillation caused by the dense integration of power electronics devices.On July 1,2015,a subsynchronous oscillation(SSO)accident occurred in the Hami wind power cluster in Xinjiang,China,which propagated across five voltage levels,resulting the trip of turbine-generators due to the activation of torsional vibration protection.At present,the SSOs caused by the dense integration of direct-drive permanent magnet synchronous generator(DPMSG)based wind farms have seriously threatened the safe and stable operation of the power system.However,the traditional negative damping oscillation theory is insufficient to reveal the mechanism of the new characteristics of the SSOs in Xinjiang.The mechanism of the DPMSG-involved SSOs is unclear.Therefore,taking the SSOs event of wind power in Xinjiang as background,this paper conducts an in-depth study on the mechanism and dynamic characteristics of DPMSG-involved SSOs based on the forced oscillation theory.The main research contents and achievements include:(1)The inherent subsynchronous characteristics of the DPMSG wind interconnected system is revealed based on the small disturbance dynamic model.The SSO accidents and recorded data in Xinjiang,China,are introduced.The new characteristics of SSOs in Xinjiang are analyzed.A small disturbance dynamic model of the DPMSG wind interconnected system is established.Based on the dynamic model,the influence of operating parameters and control parameters on the characteristics of the SSO mode is analyzed.The analysis results show that the change of wind speed has little effect on the frequency of SSO mode.The SSO mode frequency is mainly dominated by the converter control system.Meanwhile,it also shows that the negative damped oscillation theory has significant defects in revealing the time-varying characteristics of the oscillation frequency.(2)The mechanism and dynamic characteristics of DPMSG-involved forced SSOs are studied.The modulation theory is used to analyze the causes and time-varying mechanism of interharmonics from low-speed direct-drive DPMSGs.It is proposed that the low-frequency interharmonics generated by direct-drive DPMSGs are the excitation source in the wind power interconnected system,and the wind turbine control system is the forced oscillation system.The mechanism of forced SSOs in DPMSG wind interconnected system excited by the interharmonics is revealed.It is found that the forced DPMSG-involved SSOs are caused by the interaction between the interharmonics and the weakly damped SSO mode of the forced system when their frequencies are close.The forced oscillation theory is used to effectively explain the new characteristics of SSOs in Xinjiang.It is revealed that the wind SSOs in Xinjiang is essentially the forced SSO caused by interharmonics from DPMSGs.(3)The response characteristics of the DPMSG grid-side converter(GSC)to interharmonic disturbances are studied.The subsynchronous equivalent impedance model of the converter is established.The effects of the phase-locked loop(PLL)parameters,DC-link voltage control parameters and current control parameters of the control system on the interharmonic response characteristics are analyzed.The dominant control parameters and changing rules of forced SSOs caused by interharmonic amplification are revealed.Studies have shown that the parameters of the PLL and the DC-link voltage control parameters have significant effects on the interharmonic response characteristics of different frequency bands,while the parameters of the current control parameters are less affected.