考虑直流侧动态的跟网型变换器稳定性分析

目前大部分关于并网变换器的研究忽略直流侧动态，在直流侧使用恒定电压源，一定程度上影响小干扰稳定性分析。本文主要考虑直流侧动态对跟网型变换器进行建模及稳定性分析。首先，分析了新能源场站并网系统直流侧可等效为电压源和受控电流源的适用条件，论证了并网变换器建模时考虑直流侧动态的必要性。随之建立了考虑直流侧动态的跟网型变换器谐波状态空间 (harmonic state-space, HSS) 阻抗模型。其次，在不同电网强度下，通过伯德判据对不同直流侧结构的系统进行稳定性分析，揭示了电网强度对跟网型变换器稳定性的影响机理。然后，分析了锁相环、电流环、滤波环节对系统阻抗特性的影响。理论分析与电磁暂态仿真结果表明弱电网条件下，锁相环与电网呈现强交互作用，降低了系统的小干扰稳定性，考虑直流侧动态的系统临界短路比更大。

Stability analysis of grid-following converter considering DC side dynamics

DC side dynamics are ignored in most of the research on grid-connected converter, which is dealt as a constant voltage source. In this case, the analysis result of small-signal stability is affected to some extent. In this paper, the impedance model of grid-following converter is reconstructed, then followed by stability analysis of the system considering DC side dynamics. First, the applicable conditions are analyzed, that is to say, DC side is equivalent whether to voltage source or controlled current source. It is demonstrated that the DC side dynamics must be considered in case of small-signal stability analysis. Then, the impedance model of the grid-following converter is established based on harmonic state-space (HSS). Second, the stability of the system with different DC side structures is analyzed by Bode criterion under different grid strengths. Subsequently, it is revealed that the influence mechanism of grid strength on the stability of grid-following converter. In addition, the influence of different links on impedance characteristics of the system is analyzed, including phase-locked loop, current loop and filter. Theoretical analysis and electromagnetic transient simulation results show that under the condition of weak grid, the interaction between phase-locked loop and grid is strengthened, which reduces the small-signal stability of the system. Moreover, the critical short-circuit ratio of the system considering the DC side dynamics is larger.