弱交流电网下不同位置的直驱风机对次同步振荡特性影响的比较分析

Comparative Analysis of the Influence of PMSG at Different Positions on Subsynchronous Oscillation Characteristics in Weak AC Power Grid

针对直驱风电场并入弱交流电网的次同步振荡(Sub-Synchronous Oscillation,SSO)问题,现有研究多将整个风电场等值为单机模型进行研究。但实际风电场呈空间分布,且相距较远,而单机模型无法反映这一特性。针对这一问题,将整个风电场分为两个子风电场,分别等效成一台直驱风机,建立双机并入弱交流电网的小信号数学模型,并在PSCAD/EMTDC中建立仿真模型。通过特征值分析和时域仿真,分析每台风机的风速、输出功率和输电线路电抗对系统SSO特性的影响。特征值分析和时域仿真结果表明:当风速相同时,不同位置的风机对系统SSO特性影响程度不同,汇流母线近端的风机对系统稳定性的影响大于远端风机;当整个风电场的开机数为定值,远端子风电场开机数越大,系统稳定性越好;短路比越大系统越稳定,并网距离近的等值子风电场对系统SSO特性影响更大,且当风电场外部电网阻抗和内部线路阻抗之比达到一定值时,可忽略内部线路阻抗对系统的影响。

Existing studies of sub-synchronous oscillation(SSO)caused by integrating direct-drive wind farms into a weak AC power grid mainly simulate the entire wind farm with a stand-alone model.However,such model is not able to reflect the long-distance distribution of actual wind farms.Therefore,this paper divides the entire wind farm into two sub-wind farms.Each of them is equivalent to a PMSG.We then establish a small-signal mathematical model with double-PMSG integrated into a weak AC power grid,and a simulation model in PSCAD/EMTDC.By means of eigenvalue analysis and time-domain simulation,we analyze the influence of each fan's wind speed,output power,and transmission line reactance on the SSO characteristics of the system.The analysis results show that when the wind speed is the same,the fans at different positions influence the SSO characteristics of the system in various degree.The fans at the near end of the busbar have greater impact on the system stability than the remote fans do.When the starting number of the entire wind farm stay at a fixed value,the greater the starting number of remote sub-wind farms is,the more stable the system remains.The higher the short-circuit ratio is,the more stable the system is.And the equivalent sub-wind farms with a close grid connection have greater impact on the SSO characteristics of the system.When the ratio of the external grid impedance to the internal line impedance of the wind farm reaches a certain value,the impact of internal line impedance on the system can be ignored.