Study On Analysis Method For SSO Caused By PMSG Connected To Weak AC System

The subsynchronous oscillation(SSO)may occur when the wind power is integrated to the weak AC system.The SSO events,represented by SSO of wind power in Hami Shanbei region of Xinjiang in July 2015 and Yandun Jingxia region of Xinjiang in September 2017,show that wind SSO will damage the safety of equipment in new energy station,threaten the safe and stable operation of the system and affect the consumption of the new energy.Meanwhile,the converter controller of wind turbine generator has multiple fast response control loops.The SSO caused by the converter controller is essentially different from the traditional SSO related to the shaft of the turbine generator.In this dissertation,the typical example of SSO caused by permanent magnet synchronous generator(PMSG)connected to weak AC system is presented.The SSO analysis method on risk screening and stability analysis are systematically studied.The main work and innovation of the dissertation are detailed as follows:(1)The calculation method of aggregated short circuit ratio(ASCR)is proposed for the risk screening of SSO caused by grid-connected large-scale wind farms.The problems of conventional SCR in risk analysis of SSO in actual wind farm are analyzed and pointed out.A calculation method and its formula of ASCR considering multiple factors is proposed.The calculation results using actual power grid data show that the proposed calculation method can reflect the effect of the number of wind turbine generators,grid conditions and the control system on the SSO risk.It provides a theoretical basis for rapid risk screening of SSO caused by grid-connected wind farms and selecting high-risk areas for further fine stability analysis and control.(2)The generating mechanism of multiple sub-/super-synchronous frequency components in the wind power collecting zone is revealed.The amplitude calculation method is proposed for different frequency signals.In this dissertation,the expressions of output signals are derived when the subsynchronous frequency component is fed into different blocks of control system as input signal.The cause of multiple frequency components produced by the signal fed into converter control system is clarified.The closed-loop transfer function with two inputs and two outputs of converter controller is further deduced.The amplitude calculation of the dominant sub-/super-synchronous frequency component is realized,so as to implement the suppression measures for the dominant frequency component.The theoretical analysis is validated by the time domain simulation and analysis results of data recorded in a new energy collecting area in China.(3)The method for analyzing the SSO stability of PMSG connected to the weak AC system is proposed.On the basis of deducing the closed-loop transfer function,a pole-based stability analysis method for analyzing the system SSO is proposed,and the stability of SSO in the new energy collecting zone is analyzed using the actual data.Meanwhile,the calculation results point out that under weak AC system the PLL?current loop and voltage feed-forward are coupled and interconnected to increase the risk of SSO.(4)The calculating method for the SSO parameter stability region for converter controller of PMSG is proposed.Using the closed-loop transfer function,for the different AC system strength,the parameters stability region of SSO considering the PLL,current loop and DC-link capacitor is constructed.The interaction of the parameter stable interval and the effect of system strength on the parameters stability region are further analyzed.The purpose is to provide theoretical support for whether the SSO in the wind collecting zone can be alleviated by optimizing and adjusting the converter controller parameters of wind power.