单边限幅参与的并网VSC系统次同步振荡近似分析

针对电力电子设备引起的次同步振荡问题,现有研究集中在无法计及限幅的小信号模型分析或考虑限幅对称的近似分析,但极少考虑不对称的单边限幅的影响。针对不对称的单边d轴电流限幅(AU-d-CL,或上电流饱和限幅)参与的并网电压源换流器(voltage source converter,VSC)系统的次同步振荡现象,采用单边限幅的描述函数和广义Nyquist判据进行近似分析。具体地,首先,建立了并网VSC系统的简化模型,分析了其小扰动稳定性,并分析了AU-d-CL参与的负阻尼振荡现象。其次,给出了VSC系统的频域模型和AU-d-CL的描述函数。最后,结合AU-d-CL的描述函数和广义Nyquist判据,近似计算了不同参数和限幅值时的振荡幅值和频率,结果表明计算结果和仿真结果基本一致,可解释次同步振荡在限幅值改变时,频率基本不变,振荡幅值变化的现象。

Approximate Analysis of Sub-synchronous Oscillation for Grid-connected VSC System Induced by Unilateral Limit

To the sub-synchronous oscillation(SSO)caused by the power electronic equipment,the current researches mainly focus on the small-signal model,which does not account for limit,or the approximate model considering symmetry limits,but they rarely consider the impact of asymmetric limit.In this paper,for the sub-synchronous oscillation phenomenon of the voltage source converter(VSC)system induced by the asymmetric unilateral d-axis current limit(AU-d-CL,or the upper saturation limit),the describing function of the unilateral limit and the generalized Nyquist criterion are used for the approximate analysis.Specifically speaking,firstly,a simplified model of the grid-connected VSC system is established.The small-signal stability property of the grid connected VSC system is analyzed,and the phenomenon of the negative damping SSO induced by AU-d-CL is presented.Furthermore,the frequency-domain model of th eVSC system and the describing function of the AU-d-CL are deduced.Finally,combined with the describing function and the generalized Nyquist criterion,the amplitude and frequency of the SSO with different parameters and limit values are approximately estimated,the results of which are basically consistent with the simulation results.It may explain the phenomenon that the amplitude of the SSO varies when the limit values vary while the frequency remains unchanged.