Modeling Type-4 Wind in Weak Grids

An existing wind power plant at ERCOT experienced poorly damped and undamped low-frequency oscillations at 3 ~ 4 Hz under weak grid condition. The objective of this paper is to shed the insight of the oscillation mechanism through linear system analysis. Two simplified models are developed and compared against a detailed model for Type-4 wind with weak grid interconnection. The detailed model includes grid-side converter's outer power/voltage control, inner current controls, phase-locked loop (PLL), and transmission line electromagnetic dynamics. The first simplified model uses a first-order delay to replace the current control loop and ignores the transmission line dynamics and PLL. The second simplified model uses the same assumptions except that PLL is considered. Linearized system block diagrams for the two simplified models are derived and compared. The mechanism of the low-frequency oscillations are explained using eigenvalue analysis and the Root-Locus method. The root causes are identified as weak grid, high wind power export, low voltage, and low PLL bandwidth. Further, the simplified model considering PLL dynamics is more accurate in low-frequency oscillation mode identification and system stability prediction.