极弱电网下直驱风电并网变流器小信号建模及稳定性运行策略分析

Small signal modeling and stable operation strategy analysis of direct drive wind power grid-connected converter in extremely weak power grid

极弱电网下,直驱风电并网变流器各控制环路动态交互、满功率运行会改变并网点电压,导致不能稳定运行。对此,建立了考虑控制延时环节的全阶状态空间小信号模型,并采用特征值分析方法分析系统稳定性。研究表明系统稳定性随电网强度降低呈非线性变化,存在不连续稳定区间,且控制延时对不同振荡模式的阻尼比影响也呈非线性,部分阻尼比变大,部分阻尼比变小;同时,得到了参与电压、电流和锁相环之间交互作用的主导状态/控制变量,其参与度随电网强度减弱而加剧,进一步分析了各控制器参数对系统稳定性的影响规律,拉开控制带宽,避免频带交叠,将有助于提高系统稳定性;通过对比分析直驱风电系统在定交流电压控制和电压下垂控制下的适用性,发现定交流电压控制更适合极弱电网工况。通过MATLAB/Simulink仿真验证了理论分析的正确性。

In the extremely weak power grid,the dynamic interaction of the control loops of the direct drive wind power grid-connected converter and full power operation will change the grid-connected point voltage,resulting in unstable operation.In this regard,a full-order state space small signal model considering control delay is established,and the eigenvalue analysis method is used to analyze the system stability.The research shows that the system stability changes nonlinearly with the decrease of power grid strength,and there exist discontinuous stable regions.Moreover,the control delay has a non-linear effect on the damping ratio of different oscillation modes,some damping ratios become larger,while some damping ratios become smaller.At the same time,the dominant state/control variable participating in the interaction between voltage,current and phase-locked loop is obtained,and their participation increases with the decrease of power grid strength.The influence principal of each controller parameter on the system stability is further analyzed,and it will be helpful to improve the system stability to undraw the control bandwidth and avoid frequency band overlap.By comparing and analyzing the applicability of direct drive wind power system under constant AC voltage control and voltage droop control,it is found that constant AC voltage control is more suitable for the working conditions of extremely weak grid.The correctness of the theoretical analysis is verified by MATLAB/Simulink simulation.