Improved Grid Impedance Compensation for Phase-Locked Loop to Stabilize the Very-Weak-Grid Connection of VSIs

Abstract

Voltage source inverters (VSIs) with vector control based on phase-locked loop (PLL) suffer instability when connecting to a very weak AC grid (short circuit ratio (SCR)<1.3). The conventional inductive grid impedance compensation for the PLL by virtually reducing the grid impedance can stabilize this connection. However, the analysis in this paper indicates that its stabilization effectiveness is sensitive to grid impedance variance and, indeed, overcompensation causes the PLL instability. Therefore, in this paper, an improved grid impedance compensation for the PLL is proposed to achieve the same stabilization for very-weak-grid connection and possess a good tolerance of grid impedance variance and overcompensation. A comprehensive small-signal model of the VSI using the proposed PLL's grid impedance compensation is derived for stability analysis and parameter design. The time-domain simulation for this VSI is built to validate the stability analysis. Comparison studies for both proposed and conventional PLL's grid impedance compensation are conducted including the stability effectiveness, VSI performance and grid impedance variance.

Analysis and Dynamic Stabilization of a High-Voltage VSC-MTDC Grid With DC Power Flow Controller Connected to Extremely Weak AC Systems

“…9(a), given that unstable MFM has a frequency of 450 rad/s, 𝜔 0 can be set to 450 rad/s. To integarete the MFO compensator dynamics into the system's state space model (27), two additional states are added based on (29).…”

“…However, the PSC approach can compromise the converter performance during fault conditions due to the absence of direct current control, necessitating a switch back to the vector control strategy to limit converter current [27], [28]. Moreover, it can result in instability and sub-synchronous resonance in stiff grids due to voltage conflict, as reported in [29], [30]. Additionally, practical applications of the PSC for regulating HVDC converters on a large scale remain limited [31].…”