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Small-Signal Stability Analysis and Enhanced Control Strategy for VSC System During Weak-Grid Asymmetric Faults

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.