Analyses of Factors Affecting Transient Synchronous Stability of Grid-connected Systems with Full-power Converters

Photovoltaic and wind turbines connected to weak grids may trigger transient synchronous stability problems under severe fault conditions due to the coupling effect between the converter output current, grid impedance and phase-locked loop (PLL), leading to out-of-step and off-grid of new energy equipment. In this paper, a nonlinear model suitable for transient synchronous stability analysis is established for the grid-connected system with full-power converter to describe the swinging process of the converter's virtual power angle; and the transient synchronous instability mechanism of the converter is analyzed with reference to the equal-area criterion (EAC), as well as the influences of the fault conditions, grid parameters, the converter's PLL parameters and output reactive current. Results show that the higher the voltage amplitude of fault point, or the larger the system short-circuit ratio (SCR), or the further the distance from the converter to the fault point, then the better the transient synchronous stability. Also, reducing the integration coefficient of the PLL, and inductive reactive power output by the converter in the initial state or during the fault period can help to increase the transient stability margin. In addition, the phase-angle jumping at the fault point is unfavorable to the transient synchronous stability in general.