This letter presents the discovery of a new type oscillations associated with inverter-based resources (IBRs) in weak grids. While typical weak grid oscillations become worse when the grid strength decreases and the power exporting level increases, this new type of oscillations appear when the grid becomes weaker and the power exporting level decreases. Electromagnetic transient (EMT) simulation results are presented, along with eigenvalue loci for a varying power. The notable signatures of such oscillations are also remarked. In further analysis, a feedback system consisting of two blocks is derived with one block associated with the phase-locked-loop (PLL) and the other associated with the rest of the system. This feedback system quantitatively demonstrates why insufficient damping in the PLL may be an issue for stability. Open-loop analysis shows that a larger phase lag in angle tracking in the low-frequency range due to high power exporting level at the weak grid condition is beneficial to the open-loop poles introduced by the PLL. And this is the reason why such oscillations may appear when power is ramping down.

中文翻译：

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一种新型弱电网 IBR 振荡

这封信介绍了一种与弱电网中基于逆变器的资源 (IBR) 相关的新型振荡的发现。典型的弱电网振荡随着电网强度降低和功率输出水平增加而变得更严重，而这种新型振荡出现在电网变弱和功率输出水平降低时。给出了电磁瞬态 (EMT) 仿真结果，以及不同功率的特征值轨迹。这种振荡的显着特征也被指出。在进一步分析中，推导出一个由两个模块组成的反馈系统，其中一个模块与锁相环 (PLL) 相关联，另一个与系统的其余部分相关联。这个反馈系统定量地证明了为什么 PLL 中的阻尼不足可能是稳定性问题。开环分析表明，由于弱电网条件下的高功率输出电平，低频范围内角度跟踪的较大相位滞后有利于锁相环引入的开环极点。这就是为什么当功率下降时可能会出现这种振荡的原因。