Wind in Weak Grids: 4 Hz or 30 Hz Oscillations?

With wind being integrated into weak grids, Texas sees 4 Hz oscillations while China's west region sees 30 Hz oscillations. While both phenomena have been studied and are claimed to be due to weak grid wind integration, it is valid to ask this question: What causes the significant difference in oscillation frequencies? In this letter, we offer an explanation and provide evidences through dynamic modeling and eigenvalue analysis. One of the possible reasons is identified as phase-locked-loop parameters. Furthermore, both the low-frequency and subsynchronous-frequency modes exist at the same time for Type-4 wind with dc-link voltage control.