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.

I. Introduction

The power system is going through a prominent revolution marked by the increasing penetration of power electronics devices [1], [2], due to the wide application of variable renewable sources, high voltage direct current transmission (HVDC), and power electronics-based adapters of the loads. From a system perspective, the power electronics converters can change the system characteristics significantly. That power electronic converters replace traditional synchronous generators can increase the equivalent impedance of the grid, resulting in weak grids [3]. From a local viewpoint, this system change can in turn influence the operation of the power electronics device, which significantly increases its nonlinear behaviors and even deteriorates the stability [4], [5].