Analysis on the Low-Frequency Bifurcation Phenomena of Weak-Grid-Tied VSCs

Voltage source converters (VSCs) have been increasingly applied in power systems for the past decade, whose interactions with the grid give rise to a variety of complex behaviors. In this paper, we study the low-frequency bifurcation phenomena of weak-grid-tied VSCs considering the phase-locked loop (PLL). The linearized state-space representation of the weak-grid-tied VSC system is firstly obtained, on which the eigenvalue-based analysis is performed. The loci of the eigenvalues show that the weak-grid-tied VSC system loses stability through a Hopf bifurcation and that improper grid parameters-the active power and the equivalent impedance of the AC system seen from the point of common coupling (PCC), can contribute to the occurrence of the bifurcation behavior of the system. Time-domain simulation results validate the correctness of the analysis and conclusions.