Small-signal stability issues of the voltage source converters (VSC) connected weak-network during asymmetric faults is currently rarely studied. This paper develops mathematic model of VSC system during weak-grid asymmetric faults, which reveals dynamic coupling between the phase-locked loop (PLL) and both positive-sequence (PS) control system and negative-sequence (NS) control system. Based on the deduced model, the generalized Nyquist criterion (GNC) method and modal analysis are adopted to study the small-signal stability of the VSC system during asymmetric fault steady-state. The analysis demonstrates that the small-signal stability of the system during asymmetric faults is dominated by the PLL mode, and the dynamic coupling between PLL and current control loops (CCLs) will produce negative damping, which will introduce the small-signal instability risk to the system. Moreover, an improved control strategy is designed for enhancing the small-signal stability of the VSC system during weak-grid asymmetric faults, by compensating the disturbance terms related to PLL dynamic to suppress the negative damping introduced by coupling between the PLL and both the PS and NS CCLs. Finally, this paper verifies the effectiveness of the proposed control strategy through simulation and experiment.

中文翻译：

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弱网不对称故障时VSC系统小信号稳定性分析及增强控制策略


目前对连接弱网的电压源换流器（VSC）在不对称故障期间的小信号稳定性问题的研究很少。本文建立了弱电网不对称故障时VSC系统的数学模型，揭示了锁相环（PLL）与正序（PS）控制系统和负序（NS）控制系统之间的动态耦合。基于推导的模型，采用广义奈奎斯特准则（GNC）方法和模态分析来研究VSC系统在非对称故障稳态下的小信号稳定性。分析表明，不对称故障时系统的小信号稳定性主要由PLL模式决定，PLL与电流控制环(CCL)之间的动态耦合会产生负阻尼，从而给系统带来小信号不稳定风险。系统。此外，设计了一种改进的控制策略，通过补偿与 PLL 动态相关的扰动项来抑制 PLL 和 PS 之间的耦合引入的负阻尼，以增强 VSC 系统在弱电网不对称故障期间的小信号稳定性。和 NS CCL。最后，本文通过仿真和实验验证了所提出控制策略的有效性。